On Monday, momentum built among a small group of Republican senators to call President Donald Trump’s former national security adviser, John Bolton, as part of the impeachment trial following a report from the New York Times that Bolton confirms in his forthcoming book the existence of a quid pro quo at the center of the Ukraine scandal. While Trump’s defense team declined to respond to the allegation on Monday, describing it as “speculation,” the team did warn senators that if they attempted to subpoena Bolton or other witnesses they would be in for a lengthy court fight over executive privilege.

“When the House managers say they didn’t have time to litigate, they didn’t have time to come to the courts, but they now come to this chamber and say, ‘This chamber should issue some more subpoenas, this chamber should get some witnesses that we didn’t bother to fight about,’ what do you think will happen then?” deputy counsel to the president Patrick Philbin warned. “That there won’t be similar assertions of privilege and immunity, that there wouldn’t be litigation about that?”

The threat to fight a Senate subpoena for testimony from Bolton in court may be a way to pressure wavering senators to avoid voting for witness testimony, lest it lead to a drawn-out and politically damaging legal battle in the midst of a presidential campaign. The argument that executive privilege protects Bolton’s testimony, though, would likely fail in court—assuming a court would even entertain it. It would also presumably fail with a majority of the Senate if a majority voted to subpoena Bolton in the first place. In fact, it is even weaker than the arguments put forth by Richard Nixon’s administration in Nixon v. United States, the 8–0 decision by the Supreme Court to force the release of the Nixon tapes at the center of the Watergate scandal.

According to numerous sources, Mr. Bolton’s manuscript asserts that Trump informed him personally of his intent to freeze security aid to Ukraine in order to secure that country’s help with investigations into Democrats, including Joe and Hunter Biden.

Bolton’s apparent disclosure can thus provide a so-far missing piece of the Ukraine story: a firsthand account of the president’s articulation of his motivations in withholding military aid. (Acting White House chief of staff Mick Mulvaney said that Trump had withheld aid to secure an investigation into the hack of the 2016 DNC server, but later attempted to walk back those remarks.) Trump’s defenders have harped on the absence of such evidence as a major vulnerability in the case for impeachment. The news stories ratchet up pressure on Senate Republicans to approve calling Bolton as an impeachment trial witness. Bolton has already said he would testify.

“Executive privilege” is probably best understood as an umbrella term covering a number of legal privileges that presidents enjoy in order to avoid the compulsory disclosure of executive branch information. United States v. Nixon, the Court’s 1974 decision in the Watergate tapes case, certified that one aspect of that privilege—the privilege for presidential communications—has constitutional roots.

Presidents, the court noted, need unfettered access to the candid advice of others “in the process of shaping policies and making decisions.” Protecting the president’s privacy to enable the exchange of candid views “is fundamental to the operation of Government, and inextricably rooted in the separation of powers under the Constitution.”

The Nixon Court went on to hold, however, that the presidential communications privilege is not absolute. It must give way, the court determined, when overbalanced by the “legitimate needs” of another branch of government—in Nixon, the judiciary—in fulfilling its own constitutional role. In the case of the tapes, “the allowance of the privilege to withhold evidence that is demonstrably relevant in a criminal trial would cut deeply into the guarantee of due process of law and gravely impair the basic function of the court.”

Although the outcome in Nixon was true to the spirit of checks and balances, one aspect of the court’s balancing was puzzling. A number of privileges that courts routinely respect in the context of criminal trials necessarily prevent judicial access to truth-related evidence. Spousal privilege, doctor-patient privilege, cleric-penitent privilege, and attorney-client privilege all potentially interfere with the disclosure of evidence that may be not only relevant, but even critical to a criminal prosecution.

It is logical to reach the conclusion that the rule-of-law interests that the court tacitly found to have legitimately outweighed the claim of privilege in Nixon were not merely the due process interests of the Watergate defendants. Rather, they were the interest of the Supreme Court in not being party to a presidential cover-up and the interest of Congress, though not a party to the suit, in securing the tapes for its impeachment investigation. Thus, the case for the Nixon precedent to hold in the Trump impeachment is strong.

Indeed, with regard to Bolton’s potential testimony on Ukraine, the balancing of two branches’ competing interests is actually even more straightforward than it was in Nixon. Trump may argue his interest here is greater than Nixon’s because Bolton’s potential testimony regarding Ukraine involves foreign diplomacy—a core function of the executive branch. In this case, however, the ordinary weightiness of a foreign affairs privilege is profoundly reduced by the fact that the information in question is already largely in the public’s hands. The president’s voluntary release of a summary of his phone call with Ukrainian president Volodymyr Zelensky all but conclusively waives any argument that testimony shedding further light on that phone call would compromise U.S. foreign policy. Trump’s tweet accusing Bolton of lying in his account further cements the topic’s appropriate availability as of now for public discussion.

On the other hand, the Senate’s need for Bolton’s testimony, should a subpoena be issued, is constitutionally at its zenith. It addresses what the president’s defenders have themselves identified as a major point of contention in proving or disproving the articles of impeachment. The process that Bolton’s testimony would inform is Congress’s foundational check on presidential abuse of power. To deny the Senate access to Mr. Bolton’s testimony would, in the words of United States v. Nixon, “gravely impair the basic function of” the Senate.

Impeachment is so fundamental to checks and balances that its availability is commonly invoked as a reason not to subject presidents to other forms of accountability that ordinary citizens shoulder. For example, the Supreme Court has cited the availability of impeachment as a reason why presidents should not be held accountable in damages for wrongful official actions. In a memorandum asserting that a president’s discretionary official acts can never be the basis for a criminal prosecution, the once-and-future Attorney General William Barr wrote: “[U]nder the Framers’ plan, the determination whether the President is making decisions based on ‘improper’ motives or whether he is ‘faithfully’ discharging his responsibilities is left to the People, through the election process, and the Congress, through the Impeachment process.”

The logic here is straightforward: Without an effective impeachment process, the government’s capacity to hold presidents to account for corrupt abuses of power would be eviscerated. Should Trump try to block Bolton’s testimony on the basis of executive privilege, the Senate would be on the strongest possible ground in holding paramount its need for disclosure.