WASHINGTON — President Trump said Friday he would invoke executive privilege to try to block his former national security advisor from testifying in a Senate impeachment trial as Speaker Nancy Pelosi said the House will be prepared to move articles of impeachment to the Senate next week.

Trump’s statement, in an interview with Fox News host Laura Ingraham, marked the first time he openly said he would try to block John Bolton from testifying if the Senate were to subpoena his former advisor.

“I think you have to, for the sake of the office,” he said when Ingraham asked whether he would try to use executive privilege to prevent such testimony.

“You can’t be in the White House as president, future, I’m talking about future — any future presidents — and have a security advisor, anybody having to do with security, and legal and other things” testify, Trump said.

Whether the Senate would have to accept an executive privilege claim in an impeachment trial is an issue that has never been resolved.

Trump’s statement came as Pelosi signaled an end to a standoff between House Democrats and Senate Republicans that has delayed a Senate trial of the case.

“I have asked Judiciary Committee Chairman [Jerrold] Nadler to be prepared to bring to the Floor next week a resolution to appoint managers and transmit articles of impeachment to the Senate,” Pelosi (D-San Francisco) wrote in a letter to lawmakers Friday. “I will be consulting with you at our Tuesday House Democratic Caucus meeting on how we proceed further.”

The movement of the articles from the House to the Senate will kick off several days of pretrial proceedings in the Senate, including formal notification of the president. Within days, the Senate would be expected to begin a trial that will end in a vote on whether to convict Trump and remove him from office.

It is unclear exactly when a trial would begin, although the week of Jan. 21 seems most likely. The trial is expected to last at least two weeks, forcing the senators who are also 2020 presidential candidates to remain in Washington ahead of the Iowa caucuses next month.

The president’s fate is all but certain: The Constitution requires two-thirds of the Senate — or 67 senators — to support removal. There are only 47 Democrats, and no Republican has indicated he or she is ready to cross the aisle to support convicting the president.

The issue of whether the Senate will hear live witnesses during the trial has been the most contentious part of the debate in the weeks since the House voted Dec. 18 to impeach Trump.

On Monday, Bolton significantly heated up that debate by saying he would be willing to testify if subpoenaed by the Senate.

Witnesses who testified in the House proceedings said that Bolton had likened Trump’s effort to pressure Ukraine to a “drug deal,” and Democrats believe his testimony could be damaging to Trump, even if it doesn’t sway Republican senators to vote to convict him.

Some Senate Republicans have indicated they might be interested in hearing Bolton testify, but none have been willing to break ranks with Senate Majority Leader Mitch McConnell and commit to a subpoena at this point.

On Friday, however, Sen. Susan Collins (R-Maine) told reporters in her home state that she’s speaking with a “fairly small group” of fellow Republican senators about potentially allowing House Democrats to call witnesses once they have finished presenting their arguments.

“We should be completely open to calling witnesses,” she said, according to the Bangor Daily News.

“I am hopeful that we can reach an agreement on how to proceed with the trial that will allow the opportunity for both the House and the president’s counsel if they choose to do so.” Collins had previously expressed interest in hearing from Bolton.

Pelosi had held the articles in hopes of getting more favorable terms for a Senate trial, including a commitment in advance for subpoenas of witnesses and documents. She was hoping to build pressure on McConnell (R-Ky.) and his Republican colleagues.

But Senate Republicans refused to back down from their plan to punt any decision on witnesses until after the trial gets underway, leaving Democrats with little leverage.

This week, McConnell announced that he has enough Republican votes to adopt trial rules that don’t immediately allow witnesses.

His plan would call for a decision to be made on whether to subpoena witnesses after arguments from House Democrats, acting as prosecutors, and the president’s lawyers. He noted that a similar process was used during President Clinton’s impeachment trial.

Once it became clear that McConnell wouldn’t budge on witnesses, Pelosi demanded that McConnell first release the text of the rules. McConnell has not done so.

In a three-week standoff between two of Congress’ most powerful leaders, it was Pelosi who blinked first, indicating that she would transmit the articles in exchange for nothing from McConnell.

Pelosi’s allies, nevertheless, called her move a win, arguing that they were able to shape public perception of the Senate trial as being unfairly tilted in the president’s favor. Democrats have accused Republicans of setting up a sham trial designed to deliver victory to Trump by refusing to examine potential new evidence and resisting offers for witness testimony.

Senate Minority Leader Charles E. Schumer of New York said Pelosi’s delay allowed Democrats to shine a light on what they view as an unfair process in the GOP-controlled Senate.

“I think she’s done a very, very good job and it’s helped our case,” Schumer said, predicting that Republicans may support a mid-trial vote on issuing subpoenas.

In recent days, a handful of Democrats voiced skepticism of Pelosi’s plan, saying that they will have more leverage to publicly pressure Republicans once the trial is underway.

And with McConnell publicly locking up the votes to proceed to trial with out Democratic support, many Democrats wanted to merely get on with a trial.

On the other side of the aisle, Pelosi’s decision to hold on to the impeachment resolution deeply frustrated Senate Republicans, even those most inclined to hear the Democrats’ case. Both Collins and Sen. Lisa Murkowski (R-Alaska), perhaps the Republican most likely to buck her party, have expressed skepticism about the delay.

“Speaker Pelosi threw the United States Congress into unnecessary chaos with this pointless delay,” said Sen. Charles E. Grassley (R-Iowa).

“From the beginning, it’s been unclear what the goal of this hurry-up-and-wait tactic was or what the country stood to gain. We now know the answer was nothing. We’ve had three needless weeks of uncertainty and confusion, causing even more division.”

McConnell has shown particular disdain for Pelosi’s delay and what he described as the House’s attempt to run the Senate, tapping into long-held friction between the chambers.

“There will be no haggling with the House over Senate procedure,” he said.

“We will not cede our authority to try this impeachment. The House Democrats’ turn is over. The Senate has made its decision.”