Senate Minority Leader Charles Schumer (D-N.Y.) quickly seized on former national security adviser John Bolton's decision to testify, if subpoenaed, as part of President Trump's impeachment trial, arguing on Monday that it puts new pressure on Republicans to support calling witnesses.

"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 in a statement.

"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," Schumer added.

Bolton is one of four witnesses that Senate Democrats want to call as part of Trump's impeachment trial.

Bolton did not appear before the House and was not subpoenaed as part of the impeachment inquiry after his lawyers made clear he would not appear without a subpoena and that he would want a court to resolve the question of whether he could be forced to testify.

Speaker Nancy Pelosi (D-Calif.) on Monday also called on Trump and Senate Majority Leader Mitch McConnell (R-Ky.) to allow for witnesses as part of the impeachment trial in the upper chamber in the wake of Bolton's offer.

"The Senate cannot be complicit in the President's cover-up," she said in a tweet.

Bolton announced in a statement on Monday that he would testify if he is subpoenaed by the Senate, a decision that immediately sent shockwaves through Washington.

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

Bolton did not indicate what he was prepared to tell senators if he is called to testify.

He witnessed key moments leading up to and following the July 25 call during which Trump asked Ukrainian President Volodymyr Zelensky to investigate a debunked theory about Kyiv's involvement in the 2016 Democratic National Committee server hack as well as former Vice President Joe Biden and his son Hunter Biden's business dealings in Ukraine.

Bolton allegedly described efforts by administration officials to press Ukraine for the investigations as a "drug deal," according to witness testimony.

It would take 51 senators to call Bolton to testify as part of the impeachment trial. All 47 Democrats are expected to back subpoenaing him, meaning they'll need to win over four Republicans.

No GOP senator has specifically endorsed having Bolton testify, and McConnell has specifically said he does not believe either Trump's team or the House impeachment managers should call witnesses.

Sen. Susan Collins (R-Maine) said last week that she is "open" to witnesses but that a decision on who, if anyone, should be called should wait until after initial arguments and senators are given a chance to ask questions.