Former national security advisor John Bolton said in a statement Monday that he is willing to testify in President Donald Trump's impeachment trial if he is subpoenaed by the Senate.

Democrats are grappling with Senate Majority Leader Mitch McConnell, R-Ky., over the rules for Trump's eventual Senate trial, which will determine whether he is convicted on the House-passed articles of impeachment and removed from office.

Senate Minority Leader Chuck Schumer, D-N.Y., wants McConnell to approve four witnesses in advance — including Bolton — but McConnell has rejected Schumer's demand, saying instead that he wants the trial to closely resemble former President Bill Clinton's.

Bolton, rather than formally agree or decline to participate in the House impeachment process, opted instead to make his decision based on the outcome of a related court case. That case, which involved his former colleague Charles Kupperman, was found to be moot by a federal judge in late December.

"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," Bolton wrote in a statement posted to his political action committee's website.

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

Bolton TWEET

Neither the White House nor McConnell's office immediately responded to CNBC's questions or requests for comment on Bolton's statement.

Republicans hold a 53-47 majority in the Senate, and none of them have yet said they supported Trump's impeachment, let alone his removal from office.

Subpoenas in the Senate would need a "simple" 51-vote majority to pass.