Washington — Former national security adviser John Bolton said Monday that he's willing to testify in President Trump's impeachment trial if the Senate subpoenas him.

"I have concluded that, if the Senate issues a subpoena for my testimony, I am prepared to testify," the former top aide to the president said in a statement on the website of his political action committee.

Before releasing that statement, Bolton first called Senate Majority Leader Mitch McConnell, but he did not succeed in reaching him. Bolton did not reach out to the White House before making his statement. CNN first reported Bolton's attempt to contact McConnell.

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Bolton has firsthand knowledge of the events at the center of Mr. Trump's impeachment, namely efforts to pressure Ukraine to announce investigations that would benefit the president politically. He pushed back against the pressure campaign and at one point likened it to a "drug deal," according to earlier testimony by other officials.

The former national security adviser's statement comes a week after a federal judge scrapped a case brought by Charles Kupperman, one of his former aides, to determine whether the House could compel him to testify. Bolton, who had been watching the case, was not formally subpoenaed by the House in the impeachment inquiry, and his lawyer threatened to go to court if he was.

"The House has concluded its constitutional responsibility by adopting Articles of Impeachment related to the Ukraine matter. 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 in his statement. "Accordingly, since my testimony is once again at issue, I have had to resolve the serious competing issues as best I could, based on careful consideration and study."

The prospect of Bolton's testimony is likely to put pressure on Senate Majority Leader Mitch McConnell and Republican senators to allow witnesses at a Senate trial, a move many GOP lawmakers have resisted. Democrats in the minority would need four Republicans to join them in voting to issue a subpoena.

At least one GOP senator expressed keen interest in hearing from Bolton. Senator Mitt Romney, who has not been shy about criticizing Mr. Trump, told reporters Monday he'd "love to hear" what Bolton has to say.

"He has firsthand information and assuming articles of impeachment do reach the Senate — why I'd like to hear what he knows." Romney said he's not sure what the process is to get Bolton to testify and he's not sure that's terribly important to him.

"What's important is that we hear from him," Romney said.

Texas Republican Senator John Cornyn expressed some openness to hearing from Bolton, too. He told the "Guy Benson Show" on Fox News Radio, "I think it's entirely likely that his testimony would be helpful to the president because it would identify basically a foreign policy dispute which is reserved to the president under the Constitution."

Asked whether he'd support hearing from Bolton, Cornyn said, "I'd have to hear what he has to offer that would help eliminate the two articles of impeachment that have been issued by the House of Representatives."

The timing of a trial also remains uncertain. House Speaker Nancy Pelosi has yet to transmit the articles of impeachment to the Senate, and it's unclear if and when she will. Pelosi has demanded McConnell specify the procedures for the trial before she sends the articles to the upper chamber.

Senate Judiciary Committee Chairman and Trump ally Lindsey Graham said on Fox News that the Senate should change its rules to allow the trial to begin without Pelosi transmitting the articles.

Sara Cook contributed to this report.