Former National security adviser John Bolton leaves his home in Bethesda, Md. Tuesday, Jan. 28, 2020. President Donald Trump's…
Former National security adviser John Bolton leaves his home in Bethesda, Maryland, Jan. 28, 2020.

With the U.S. Senate increasingly likely to call former National Security Adviser John Bolton to testify in President Donald Trump's impeachment trial, Trump's lawyers are almost certain to assert executive privilege to block his testimony.

Bolton poses a serious threat to Trump's defense in light of reports of Bolton's forthcoming book which corroborates one of the central allegations in the impeachment case against Trump —  that the president tried to coerce Ukraine into investigating former Vice President Joe Biden and his son, Hunter.

But if Trump's lawyers are determined to prevent Bolton from testifying, the claim of executive privilege is unlikely do the trick. Although courts have long recognized a president's right to have confidential communication with his advisers, that right is not absolute, experts say. What's more, when it comes to impeachment, legal experts say the courts will likely side with the interest of Congress in obtaining information over the president's executive privilege claims.   

In the Trump impeachment case, "the claim of executive privilege weakens substantially, because how can the Senate do its job of vetting all the available information if it can't even have access to that information?" said Mark Rozell, dean of the Schar School of Police and Government at George Mason University, and author of a definitive study on the subject.

The Washington Post first reported Tuesday that Senate Majority Leader Mitch McConnell had told other Republicans that he did not have the votes to block witnesses and documents. 

Executive privilege is a uniquely American concept. Although it's not explicitly mentioned in the U.S. Constitution, the principle has long been recognized as the right of the president and some senior government officials to withhold information from Congress, from the courts, and ultimately from the public, Rozell said.

FILE - Jay Sekulow, President Donald Trump's personal lawyer, speaks to reporters during a break in the impeachment trial of the president on charges of abuse of power and obstruction of Congress, in Washington, Jan. 24, 2020.

Executive privilege covers two types of information — national security secrets and confidential communication between the president and his advisers.

Bolton is the most sought-after of four witnesses Democrats want to testify in Trump's impeachment trial. A foreign policy hawk, Bolton was Trump's national security adviser from April 2018 to September 2019. His final months in the White House coincided with what Democrats say was a Trump campaign to solicit Ukrainian interference in the 2020 U.S. presidential election.  

In a political memoir slated for release in March, Bolton discloses that Trump told him in August he wanted to hold up $391 million in military aid to Ukraine until Ukraine's leader announced investigations into his political rivals, The New York Times reported Sunday.

Trump: 'National security' issue

Bolton has said he would testify in the Senate impeachment trial if he is subpoenaed. While it remains unclear whether the Republican-controlled Senate will ultimately vote to allow any witnesses, Trump and his lawyers have indicated they'll try to stop Bolton's testimony.

"The way I look at it, I call it national security," Trump said last week. "Executive privilege, they say. So, John would certainly fit into that. When you're a national security adviser … I just think it's very hard."

But executive privilege is not an unlimited power. That principle was established in a landmark 1974 Supreme Court ruling that forced the White House to turn over tape recordings of conversations between President Richard Nixon and his aides to a special prosecutor investigating the Watergate break-in.  

While recognizing the constitutionality of executive privilege, the high court unanimously ruled that the principle did not override the need for key evidence in a criminal trial. The White House handed over the tapes, and Nixon resigned two weeks later as the House prepared to impeach him.

The case for withholding information in an impeachment case is even weaker, said David Driesen, university professor at the Syracuse University College of Law.

In an impeachment context, "the interest in getting accurate information is just too great," Driesen said. "It doesn't make any sense to say that information that's important to an impeachment trial could be withheld based on executive privilege."

Possibility of compromise

Trump's lawyers haven't said how they plan to stop Bolton's testimony. But if the Senate subpoenas Bolton, the president's legal team could theoretically ask a federal district court to bar him from testifying on executive privilege grounds.

In theory, the case could eventually wind its way to the Supreme Court, but legal experts say that scenario is highly unlikely.

"I can't imagine him winning the case," Driesen said. "Frankly, even with this very conservative Supreme Court, it would be an immense change from the way they've approached such questions in the past."

Instead, a more likely path forward might be negotiations between Congress and the executive branch, which is how past disputes over executive privilege have been resolved.  

During opening arguments on Monday, deputy White House counsel Patrick Philbin said that when the president invokes executive privilege, "the goal is to reach a compromise."

One possible compromise: Bolton testifies, but only behind closed doors.