John Bolton, President Donald Trump's former national security adviser, announced on Monday that he's willing to testify in the impending Senate impeachment trial, even if the president opposes it.

Bolton's unexpected move took Washington by storm and is now the biggest thorn in the side of congressional Republicans.

The GOP for weeks accused House Democrats of holding hearings with witnesses who provided only "hearsay" instead of those with direct knowledge of events at the center of the impeachment inquiry.

But Bolton, if he testifies, could be the president's worst nightmare. Not only does he have firsthand information about events relevant to the investigation, but as the former national security adviser, he would be the highest-profile witness to testify against Trump.

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John Bolton, President Donald Trump's former national security adviser, announced on Monday that he's willing to testify in the impending Senate impeachment trial, even if the president opposes it.

"If the Senate issues a subpoena for my testimony, I am prepared to testify," Bolton said in a statement.

The former national security adviser's unexpected statement took Washington by storm and is now the biggest thorn in the side of congressional Republicans.

Shortly after Bolton's announcement, Senate Majority Leader Mitch McConnell said the upper chamber could move forward with a trial without calling witnesses — a central request from congressional Democrats — and decide on whether to do so once the trial is underway, similar to President Bill Clinton's impeachment proceedings in 1998.

Some more moderate and vulnerable Senate Republicans — including Mitt Romney and Lisa Murkowski — said they supported McConnell's decision to force a trial without guaranteeing that the Senate will call witnesses.

However, McConnell added that he can't move forward on the resolution determining the rules of the trial until House Speaker Nancy Pelosi transmits the two articles of impeachment against Trump to the Senate.

Pelosi, meanwhile, said she would send them over soon, but only after Senate Republicans make public the rules of procedure for Trump's trial.

Bolton's statement is a potential wild card in the impeachment fight and increases pressure on moderate and vulnerable Republican senators to take a clearer stance on whether to call witnesses.

"By wholly dismissing witnesses, even those with direct knowledge of the allegations, Republicans are demonstrating to their constituents that they are shills for the president," Rodell Mollineau, a Democratic strategist and DC-based consultant, told Insider. "That might be OK in red states, but if you are a Senate Republican from a purple state with a tough race in 2020 it will harder to justify."

Greg Sargent, a Washington Post opinion columnist, argued that if Senate Republicans succeed in acquitting Trump without hearing from key witnesses, "the whole affair will be forever stained by the indelible fact that they could only exonerate Trump by refusing to permit a full reckoning — and refusing to hear the very sort of testimony they themselves claimed to want for months."

Indeed, with Bolton, Democrats now have a key witness — someone with direct knowledge of Trump's dealings with Ukraine — ready and willing to make what could be very damaging claims against the president. And Republicans are poised to deny Bolton the chance to divulge information about what Bolton's lawyer has said are "many relevant meetings and conversations."

Trump and Senate Majority Leader Mitch McConnell. Reuters

"The testimony & evidence considered in a Senate impeachment trial should be the same testimony & evidence the House relied upon when they passed the Articles of Impeachment," Sen. Marco Rubio, a Florida Republican, tweeted on Monday. "Our job is to vote on what the House passed, not to conduct an open ended inquiry."

Republicans say they're confident the party will stay united through the trial.

"Lisa Murkowski's announcement that she supports Majority Leader McConnell's request that a vote on witnesses occur mid-trial, just as it did in the Clinton impeachment, is far more important than Bolton's announcement," Matt Mackowiak, a conservative strategist, told Insider. "Pelosi has no leverage over McConnell, and both sides know it."

Bolton could be the single most dangerous witness against Trump

Throughout the battle over impeachment, Republicans have argued that Democrats held hearings with witnesses who provided only "hearsay" rather than witnesses with direct knowledge of the events at the center of the inquiry.

Yet they refused to acknowledge that the very reason some of those direct witnesses didn't testify was that the White House itself barred them from appearing, on Trump's orders.

Bolton, if he testifies, could be the president's worst nightmare. As the former national security adviser, he would be the highest-profile witness to testify against Trump and one who held frequent meetings with him.