The impeachment power is quasi-judicial and differs fundamentally from Congress’s legislative authority. The Constitution assigns “the sole power of impeachment” to the House—the full chamber, which acts by majority vote, not by a press conference called by the Speaker. Once the House begins an impeachment inquiry, it may refer the matter to a committee to gather evidence with the aid of subpoenas. Such a process ensures the House’s political accountability, which is the key check on the use of impeachment power.

The House has followed this process every time it has tried to impeach a president. Andrew Johnson’s 1868 impeachment was predicated on formal House authorization, which passed 126-47. In 1974 the Judiciary Committee determined it needed authorization from the full House to begin an inquiry into Richard Nixon’s impeachment, which came by a 410-4 vote. The House followed the same procedure with Bill Clinton in 1998, approving a resolution 258-176, after receiving independent counsel Kenneth Starr’s report.

Mrs. Pelosi discarded this process in favor of a Trump-specific procedure without precedent in Anglo-American law. Rep. Adam Schiff’s Intelligence Committee and several other panels are questioning witnesses in secret. Mr. Schiff has defended this process by likening it to a grand jury considering whether to hand up an indictment. But while grand-jury secrecy is mandatory, House Democrats are selectively leaking information to the media, and House Republicans, who are part of the jury, are being denied subpoena authority and full access to transcripts of testimony and even impeachment-related committee documents. No grand jury has a second class of jurors excluded from full participation.

Unlike other impeachable officials, such as federal judges and executive-branch officers, the president and vice president are elected by, and accountable to, the people. The executive is also a coequal branch of government. Thus any attempt to remove the president by impeachment creates unique risks to democracy not present in any other impeachment context. Adhering to constitutional text, tradition and basic procedural guarantees of fairness is critical. These processes are indispensable bulwarks against abuse of the impeachment power, designed to preserve the separation of powers by preventing Congress from improperly removing an elected president.

House Democrats have discarded the Constitution, tradition and basic fairness merely because they hate Mr. Trump. Because the House has not properly begun impeachment proceedings, the president has no obligation to cooperate. The courts also should not enforce any purportedly impeachment-related document requests from the House. (A federal district judge held Friday that the Judiciary Committee is engaged in an impeachment inquiry and therefore must see grand-jury materials from special counsel Robert Mueller’s investigation, but that ruling will likely be overturned on appeal.) And the House cannot cure this problem simply by voting on articles of impeachment at the end of a flawed process.

The Senate’s power—and obligation—to “try all impeachments” presupposes that the House has followed a proper impeachment process and that it has assembled a reliable evidentiary basis to support its accusations. The House has conspicuously failed to do so. Fifty Republican senators have endorsed a resolution sponsored by Judiciary Committee Chairman Lindsey Graham urging the House to “vote to open a formal impeachment inquiry and provide President Trump with fundamental constitutional protections” before proceeding further. If the House fails to heed this call immediately, the Senate would be fully justified in summarily rejecting articles produced by the Pelosi-Schiff inquiry on grounds that without a lawful impeachment in the House, it has no jurisdiction to proceed.

The effort has another problem: There is no evidence on the public record that Mr. Trump has committed an impeachable offense. The Constitution permits impeachment only for “treason, bribery, or other high crimes and misdemeanors.” The Founders considered allowing impeachment on the broader grounds of “maladministration,” “neglect of duty” and “mal-practice,” but they rejected these reasons for fear of giving too much power to Congress. The phrase “high crimes and misdemeanors” includes abuses of power that do not constitute violations of criminal statutes. But its scope is limited.

Abuse of power encompasses two distinct types of behavior. First, the president can abuse his power by purporting to exercise authority not given to him by the Constitution or properly delegated by Congress—say, by imposing a new tax without congressional approval or establishing a presidential “court” to punish his opponents. Second, the president can abuse power by failing to carry out a constitutional duty—such as systematically refusing to enforce laws he disfavors. The president cannot legitimately be impeached for lawfully exercising his constitutional power.

Applying these standards to the behavior triggering current calls for impeachment, it is apparent that Mr. Trump has neither committed a crime nor abused his power. One theory is that by asking Ukrainian President Volodymyr Zelensky to investigate Kyiv’s involvement in the 2016 U.S. presidential election and potential corruption by Joe Biden and his son Hunter was unlawful “interference with an election.” There is no such crime in the federal criminal code (the same is true of “collusion”). Election-related offenses involve specific actions such as voting by aliens, fraudulent voting, buying votes and interfering with access to the polls. None of these apply here.

Nor would asking Ukraine to investigate a political rival violate campaign-finance laws, because receiving information from Ukraine did not constitute a prohibited foreign contribution. The Mueller report noted that no court has ever concluded that information is a “thing of value,” and the Justice Department has concluded that it is not. Such an interpretation would raise serious First Amendment concerns.

Equally untenable is the argument that Mr. Trump committed bribery. Federal bribery statutes require proof of a corrupt intent in the form of a quid pro quo—defined by the Supreme Court in U.S. v. Sun-Diamond Growers (1999), as a “specific intent to give or receive something of value in exchange for an official act.” There was no quid pro quo in the call. Mr. Zelensky has said he felt no pressure, and the purported quid (military aid to Ukraine) was not contingent on the alleged quo (opening an investigation), because the former materialized within weeks, while the latter—not “something of value” in any case—never did.

 

More fundamentally, the Constitution gives the president plenary authority to conduct foreign affairs and diplomacy, including broad discretion over the timing and release of appropriated funds. Many presidents have refused to spend appropriated money for military or other purposes, on grounds that it was unnecessary, unwise or incompatible with their priorities.

Thomas Jefferson impounded funds appropriated for gunboat purchases, Dwight Eisenhower impounded funds for antiballistic-missile production, John F. Kennedy impounded money for the B-70 bomber, and Richard Nixon impounded billions for highways and urban programs. Congress attempted to curtail this power with the Impoundment Control Act of 1974, but it authorizes the president to defer spending until the expiration of the fiscal year or until budgetary authority lapses, neither of which had occurred in the Ukraine case.

Presidents often delay or refuse foreign aid as diplomatic leverage, even when Congress has authorized the funds. Disbursing foreign aid—and withholding it—has historically been one of the president’s most potent foreign-policy tools, and Congress cannot impair it. Lyndon B. Johnson used the promise of financial aid to strong-arm the Philippines, Thailand and South Korea to send troops to Vietnam. The General Accounting Office (now called the Government Accountability Office) concluded that this constituted “quid pro quo assistance.” In 2013, Barack Obama, in a phone conversation with Egyptian President Abdel Fattah Al Sisi, said he would slash hundreds of millions of dollars in military and economic assistance until Cairo cooperated with U.S. counterterrorism goals. The Obama administration also withheld millions in foreign aid and imposed visa restrictions on African countries, including Uganda and Nigeria, that failed to protect gay rights.

Further, there is credible evidence that Ukraine interfered in the 2016 presidential election at the request of senior Obama administration officials. The Justice Department is investigating this as part of its broader inquiry—now a criminal investigation—into efforts to target the Trump campaign in 2016 and beyond. It is certainly legitimate for the president to ask Ukraine to cooperate.

In addition, the president’s constitutional duty to “take care that the laws be faithfully executed” implies broad discretion to investigate and prosecute crimes, even if they involve his political rivals. Investigating Americans or Ukrainians who might have violated domestic or foreign law—and seeking the assistance of other nations with such probes, pursuant to mutual legal-assistance treaties—cannot form a legitimate basis for impeachment of a president.

It’s legally irrelevant that a criminal investigation may be politically beneficial to the president. Virtually all exercises of constitutional discretion by a president affect his political interests. It would be absurd to suggest that a president’s pursuit of arms-control agreements, trade deals or climate treaties are impeachable offenses because they benefit the president or his party in an upcoming election.

Using a private party such as Rudy Giuliani to carry out diplomatic missions is neither a crime nor an abuse of power. While the State Department’s mandarins have always lamented intrusions on their bureaucratic turf, numerous U.S. presidents have tapped people to conduct foreign-policy initiatives whose job—whether in the government or private sectors—did not include foreign-policy experience or responsibility. George Washington sent Chief Justice John Jay to negotiate the “Jay Treaty” with Britain. Woodrow Wilson used American journalist Lincoln Steffens and Swedish Communist Karl Kilbom as special envoys to negotiate diplomatic relations with the Soviet Union. A close Wilson friend, Edward House, held no office but effectively served as chief U.S. negotiator at the Paris Peace Conference after World War I.

Nor is it illegal or abusive to give a diplomatic assignment to a government official whose formal institutional responsibilities do not include foreign affairs, such as the energy secretary. JFK relied on Attorney General Robert F. Kennedy to negotiate with Moscow during the Cuban missile crisis.

Although the impeachment inquiry has been conducted in secret, what we know suggests it has become a free-ranging exploration of Mr. Trump’s foreign-policy substance and process, with the committees summoning numerous State Department witnesses. Congress could properly undertake such an inquiry using its oversight authority, but by claiming that it is proceeding with an impeachment inquiry, it has forfeited this option.

 

If the House impeaches Mr. Trump because it disapproves of a lawful exercise of his presidential authority, it will in effect have accused him of maladministration. The Framers rejected that amorphous concept because it would have allowed impeachment for mere political disagreements, rendering the president a ward of Congress and destroying the executive’s status as an independent, coequal branch of government. If the House impeaches on such grounds and the Senate concludes it has jurisdiction to conduct an impeachment trial, it should focus first and foremost not on the details of Mr. Trump’s foreign policy, but on the legal question of whether the conduct alleged is an impeachable offense.

Alexis de Tocqueville observed in 1835: “A decline of public morals in the United States will probably be marked by the abuse of the power of impeachment as a means of crushing political adversaries or ejecting them from office.” What House Democrats are doing is not only unfair to Mr. Trump and a threat to all his successors. It is an attempt to overrule the constitutional process for selecting the president and thus subvert American democracy itself. For the sake of the Constitution, it must be decisively rejected. If Mr. Trump’s policies are unpopular or offensive, the remedy is up to the people, not Congress.

Mr. Rivkin and Ms. Foley practice appellate and constitutional law in Washington. He served at the Justice Department and the White House Counsel’s Office during the Reagan and George H.W. Bush Administrations. She is a professor of constitutional law at Florida International University College of Law.