Bolton prepared to testify in Trump impeachment trial if subpoenaed Bolton is one of four key witnesses Senate Democrats say must testify.

President Donald Trump's former national security adviser, John Bolton, said he is willing to testify in the Senate impeachment trial if subpoenaed, according to a statement he released Monday.

"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.

"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. I have concluded that, if the Senate issues a subpoena for my testimony, I am prepared to testify."

The White House did not immediately respond to a request for comment.

Bolton reached out to Senate Majority Leader Mitch McConnell prior to putting out the statement as a heads up, according to a source. Bolton did not inform the White House prior to the statement's release, the person said.

The Republican-controlled Senate hasn't shown any interest in asking potential witnesses in the Ukraine matter to testify, but the move certainly puts pressure on the Senate to consider hearing from one of the key players involved in President Trump's decision to put a hold on military aid to Ukraine. As part of a trial, the Senate can vote to subpoena any witness it wants and it takes a majority of 51 senators to do so.

In a letter to McConnell last month, Minority Leader Chuck Schumer asked to hear from four witnesses including Bolton, in addition to acting White House Chief of Staff Mick Mulvaney, Associate Director for National Security at the Office of Management and Budget Michael Duffey and Robert Blair, a senior adviser to Mulvaney.

Schumer, who for weeks now has railed against Senate Republicans for failing to come to an agreement to allow witnesses and crucial documents be a part of the upcoming Senate impeachment trial, hailed Bolton's statement as a victory.

“Momentum for uncovering the truth in a Senate trial continues. John Bolton correctly acknowledged that he needs to comply with a Senate subpoena to compel his testimony, if issued. It is now up to four Senate Republicans to support bringing in Mr. Bolton, and the other three witnesses, as well as the key documents we have requested to ensure all the evidence is presented at the onset of a Senate trial," Schumer said. "Given that Mr. Bolton’s lawyers have stated he has new relevant information to share, 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," he said.

Last fall, Bolton's lawyer told House lawyers in a letter that Bolton "was personally involved in many of the events, meetings, and conversations about which you have already received testimony, as well as many relevant meetings and conversations that have not yet been discussed in the testimonies thus far.”

House Speaker Nancy Pelosi weighed in as well on Twitter.

"The President & Sen. McConnell have run out of excuses. 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," she tweeted.

According to his statement, Bolton's decision to cooperate with a potential subpoena came after the separate case of former deputy national security adviser Charles Kupperman was dismissed by a federal judge last week.

In that case, Kupperman asked the courts to determine whether he was legally obligated to provide subpoenaed testimony to Congress despite instructions from the executive branch not to comply. Kupperman was represented in the case by Charles Cooper, who currently serves as Bolton's attorney.

After filing the case in the fall of last year, House Democrats withdrew their subpoena seeking testimony from Kupperman as part of the impeachment investigation and sought to have the case dismissed. Kupperman's attorney argued that his client still remained in a "Catch-22" over whether to proceed with providing testimony.

Last week, DC District Judge Richard Leon ultimately sided with the House, finding that the case did not need to proceed in absence of a standing subpoena. The House assured the judge there was no intention to reissue the subpoena to Kupperman, however Leon did suggest that if a subpoena were to be reissued, Kupperman's case could be re-opened.

"Should the winds of political fortune shift," Leon wrote, "he will undoubtedly be right back before this Court seeking a solution to a Constitutional dilemma that has long-standing political consequences."

ABC News' Mariam Khan, Benjamin Siegel, Jordyn Phelps and Ben Gittleson contributed to this report.