White House officials are confident that President Trump’s former national security adviser John Bolton will not appear as a witness during a Senate impeachment trial.

Bolton threatened to upend Republicans’ careful strategy for a speedy trial on Monday when he announced that he was willing to testify if subpoenaed. He is one of a number of witnesses with direct knowledge of Trump’s actions toward Ukraine that Democrats believe could prove crucial in understanding the arc of events that led to impeachment.

However, a senior administration source said Bolton was unlikely to be subpoenaed by the Republican-dominated Senate and that the White House would move to block his testimony if they did.

“There is a very clear case that his conversations with the president are covered by executive privilege,” a senior administration official said, adding that any legal case to settle the question could take months, making it unlikely that he would ever appear during a trial.

Bolton’s surprise intervention has confounded pundits, who have spent the past 24 hours trying to work out his motive. He was one of four administration officials blocked by the White House from testifying during House hearings.

“I have concluded that, if the Senate issues a subpoena for my testimony, I am prepared to testify,” Bolton said Monday.

Trump’s opponents welcomed Bolton’s announcement and called on Senate Republicans to ensure he had a chance to reveal what he knew about the White House's thinking on Ukraine.

“McConnell and his Republican colleagues are the only ones standing in the way of the American people hearing what John Bolton has to say," said Sean Eldridge, president of Stand Up America, which advocates for Trump's impeachment. "Will they hold a fair trial that airs the evidence against Trump, or will they enable Trump’s cover-up?”

[Byron York: ANALYSIS: Behind Bolton's decision]

Senate Majority Leader Mitch McConnell has forged an impeachment strategy with the White House aiming for a rapid resolution. Only after both sides' lawyers had set out their respective cases would they even consider calling witnesses — a similar procedure to that of President Bill Clinton's impeachment trial.

South Carolina Sen. Lindsey Graham, one of Trump’s closest Republican allies, said Bolton’s announcement changed nothing and that he was ready to vote on the evidence presented by the House. He said: “I'm ready to vote ... just like we did in Clinton — hear the House out, let the president's team speak, then people can decide based on what you heard, do you want evidence, do you want witnesses?”

Influential Texas Republican Sen. John Cornyn said both sides should make their case before deciding on witnesses.

"I wouldn't have any objection to hearing from him, but I think what we ought to do is hear from both of the parties, as we did in Clinton,” he said, “and then decide whether there's a desire on the part of 51 senators for more."

David Drucker contributed to this report.