Former national security adviser John Bolton in a surprise announcement said Monday he'd be willing to testify in the Senate's impeachment trial of President Trump — if the Senate subpoenas him.

"I have concluded that, if the Senate issues a subpoena for my testimony, I am prepared to testify," Bolton said in a statement posted on his website.

It's a big "if." Senate Majority Leader Mitch McConnell, R-Ky., has shown no inclination to call any public witnesses, and President Trump has called for a quick trial.

House Speaker Nancy Pelosi, D-Calif., has yet to send the two articles of impeachment approved by the House last month over to the Senate, nor has the House voted on the managers who will argue the case before senators. Pelosi said she is waiting for McConnell and Senate Minority Leader Chuck Schumer, D-N.Y., to agree on "fair" ground rules for the trial first.

The House attempted to get Bolton's deputy, Charles Kupperman, to testify last fall, but he was blocked by the White House. The House withdrew the subpoena after Kupperman's lawyer sued in an effort to get a court to decide the matter.

Bolton said Monday that as "it does not appear possible that a final judicial resolution of the still-unanswered Constitutional questions can be obtained before the Senate acts," he has "had to resolve the serious competing issues as best I could."

Bolton was among four potential witnesses that Democrats had hoped to call in a Senate trial.

According to testimony from National Security Council aide Fiona Hill, Bolton warned about Trump attorney Rudy Giuliani's involvement in digging up dirt on former Vice President Joe Biden and his son Hunter's activities in Ukraine. Giuliani, Bolton said, was "a hand grenade," and he said he wanted no part of the "drug deal" being cooked up by Trump's allies.

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