Mr Bolton was sacked by President Trump in September after 16 months on the job.
Bolton, who left the White House in September, said Monday that he has weighed the issues of executive privilege and after "careful consideration and study" decided that he would comply with any Senate subpoena. He declined further comment.
This doesn't necessarily mean Bolton will testify, but his willingness to answer questions about the ongoing scandal, under oath, represents a rather dramatic and unexpected curveball. However, there is a possibility that several Democratic senators will join Republicans.
"It's a Dems scam game to help with the election!"
Bolton's statement comes after Senate majority leader Mitch McConnell (R-KY) reiterated that he had no plans to invite four current and former White House officials to testify in the trial, despite requests from his minority counterpart, Senator Chuck Schumer (D- NEW YORK).
But Democrats would need only four Republicans to side with them to get the majority required to call witnesses.
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Spurs are veering into injury crisis territory, with Hugo Lloris , Danny Rose , Ben Davies and Tanguy Ndombele now out. As Middlesbrough is not a Premier League side, the Riverside Stadium is not equipped with this kind of technology.
Both Collins and Murkowski believe the precedent set during the Clinton impeachment trial should be followed.
To the degree that Schumer and Democrats can tease out more facts about the Ukraine affair, they could try to sustain press and public interest and attempt to go on hurting Trump politically, even if they can't ultimately eject him.
"I don't think it's going to be indefinite", House Intelligence Committee Chairman Adam Schiff, a Democrat, told CNN's "State of the Union" on Sunday when asked about the delay in transmitting the impeachment articles.
Instead, he criticized House Speaker Nancy Pelosi for holding onto the articles of impeachment while Senate leaders negotiate the process.
"It's now been nearly three weeks and she hasn't taken any action, " Representative John Ratcliffe, Republican of Texas, told Fox News.
The Democrat-controlled House of Representatives voted to impeach Trump on December 18 on charges of abuse of power and obstruction.
It also charged the president with obstructing Congress by directing administration officials and agencies not to cooperate with the impeachment inquiry. "Our country is doing so good".
Schumer has sparred with McConnell to try to win assurances that key Trump White House aides will be allowed to testify at the impeachment trial, which would be only the third such impeachment proceeding in USA history.