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Responses rolled in Monday from top congressional leaders and key Republican senators on former national security adviser John Bolton's announcement that he would willingly testify in the Senate's impeachment trial if issued a subpoena.

Driving the news: Sen. Mitt Romney (R-Utah) told reporters he "would like to be able to hear from John Bolton," but added: "What the process is to make that happen, I don’t have an answer for you."

Why it matters: Bolton's testimony could offer key insights to unanswered questions at the heart of the impeachment inquiry — namely, why President Trump withheld military aid to Ukraine while seeking to pressure the country to investigate his political rivals.

  • Bolton's lawyer said in November that his client was “part of many relevant meetings and conversations” in the Ukraine scandal.

The big picture: Senate Majority Leader Mitch McConnell is standing firm on his position that no witnesses should be called, arguing that it's the responsibility of the House to investigate. Just four Republican senators must vote with Democrats to reach the simple majority necessary to call witnesses.

What they're saying:

  • McConnell: The majority leader did not mention Bolton during a Senate floor speech Monday, but said: "The Senate’s unanimous bipartisan precedent from 1999 left witnesses and other mid-trial questions to the middle of the trial. House Democrats may have scrapped their own precedents to hurt President Trump but they do not call the shots in the Senate."
  • Senate Minority Leader Chuck Schumer: “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’ve requested they would make absolutely clear they are participating in a cover up."
  • House Speaker Nancy Pelosi: "The President and 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. "

Potential swing vote Republicans like Sens. Susan Collins (R-Maine) and Lisa Murkowski (R-Ala.) were non-committal about Bolton when asked by reporters, but seemed to side with McConnell in wanting to hear opening arguments before voting.

  • Collins: "I think it's difficult to decide in isolation before we have heard the opening statements. ... There are a number of witnesses that may well be appropriate for Stage 3, of which he would certainly be one."
  • Murkowski: “I want to get to the first step. The first step is trying to get articles of impeachment which we haven’t gotten yet. So, there’s a lot of people that want to hear a lot of things but you got to get to the first step first.”

Flashback: Trump tweeted in November that "the D.C. Wolves and Fake News Media are reading far too much into people being forced by Courts to testify before Congress," and that he "would actually like people to testify," including Bolton.

  • "John Bolton is a patriot and may know that I held back the money from Ukraine because it is considered a corrupt country, & I wanted to know why nearby European countries weren’t putting up money also," Trump tweeted.

Go deeper: Bolton says he will testify in impeachment trial if Senate issues subpoena

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