Bolton’s surprise offer comes as Senate Majority Leader Mitch McConnell and Minority Leader Chuck Schumer remain at an impasse over the parameters for the chamber’s trial. Schumer has been pushing McConnell to allow additional witness testimony and document production as part of the trial, but McConnell has maintained that those issues should be considered after the trial begins.

Senate Majority Leader Mitch McConnell (R-Ky.) and Minority Leader Chuck Schumer (D-N.Y.). | Erin Schaff/Pool via AP

“It is now up to four Senate Republicans to support bringing in Mr. Bolton,” Schumer said in a statement, renewing his demand for three other witnesses to appear for testimony: acting White House chief of staff Mick Mulvaney, top Mulvaney aide Robert Blair and senior budget official Michael Duffey. All three refused to appear for testimony before House impeachment investigators.

“If any Senate Republican opposes issuing subpoenas to the four witnesses and documents we have requested they would make absolutely clear they are participating in a cover up,” Schumer added, noting that Bolton’s lawyer has already said his client has information to share with investigators that has not been previously disclosed.

The statement from Bolton — who has remained relatively quiet since Trump fired him last year — hands Senate Democrats a new weapon as they seek to exert pressure on Republicans to call witnesses and seek documentary evidence to add to the House's articles of impeachment. A Senate subpoena requires at least 51 votes, and four Republicans would need to vote with Democrats.

One of those potential GOP votes, Sen. Mitt Romney of Utah, said he would “of course” want to hear from Bolton. But he stopped short of declaring he would vote in favor of subpoenaing Bolton, adding, “What’s important is that we hear from him.”

Before his announcement, Bolton, who has been described as a central witness to the alleged behavior the House described in its articles of impeachment against Trump, gave a heads up to McConnell, according a source familiar with the matter.

GOP Sen. John Cornyn of Texas did not rule out voting to subpoena Bolton but said the question of witnesses should be left for later in the trial, echoing McConnell’s position. He suggested if that occurred, it could actually help Trump’s case.

“What I think Bolton could say, this is a disagreement over the way foreign policy is being conducted. There’s no crime being judged,” Cornyn told reporters. “I have no objection to his testifying either through his deposition or some pre-recorded testimony. To me it amounts to an admission that what they’ve presented so far is pretty thin gruel.”

Bolton’s move also unleashes new and complicated constitutional questions, including whether the House will attempt to subpoena Bolton now that he has acknowledged his willingness to comply with a congressional subpoena. Bolton’s pronouncement also raises the question of whether Trump could intervene to block his testimony.

Amid the clashes between McConnell and Schumer, House Speaker Nancy Pelosi has refused to transmit the impeachment articles to the Senate. The House impeached Trump on Dec. 18, but Pelosi has said she wants to wait until the parameters of the Senate trial become clearer. Senators expect Pelosi to send the articles later this week.

“The president & Sen. McConnell have run out of excuses,” Pelosi tweeted. “They must allow key witnesses to testify, and produce the documents Trump has blocked, so Americans can see the facts for themselves. The Senate cannot be complicit in the president’s cover-up.”

Bolton was not subpoenaed as part of the House’s impeachment inquiry, and he did not say in his statement whether he would comply with a subpoena from the lower chamber. A spokeswoman for Bolton declined to comment on whether he would honor a House subpoena.

Bolton indicated that he had planned to decide whether to testify based on the outcome of a court case brought by his former deputy Charles Kupperman. Kupperman — who had been subpoenaed to testify in the House’s impeachment inquiry but was ordered by Trump not to appear — sought a federal court ruling to resolve the conflicting demands.

But the House, seeking to disentangle its impeachment push from ongoing litigation, withdrew its subpoena and promised not to punish Kupperman for refusing to testify. The White House, too, urged the court to drop the case, claiming Kupperman was immune from testifying. Last week, Judge Richard Leon agreed, ending the short-lived court battle. Bolton acknowledged that decision, saying Leon issued a “carefully reasoned opinion.”

“It now falls to the Senate to fulfill its constitutional obligation to try impeachments, and it does not appear possible that a final judicial resolution of the still-unanswered constitutional questions can be obtained before the Senate acts,” Bolton said Monday.

Bolton’s testimony would be a major break for impeachment investigators.