WASHINGTON - John Bolton complicated Senate Republicans' impeachment strategy on Monday, declaring his willingness to testify and upping the pressure on Senate Majority Leader Mitch McConnell and his party to summon the former national security adviser as a witness in President Donald Trump's trial.

Bolton last fall rebuffed House impeachment investigators' entreaties to testify about his concerns about Trump's demands that Ukraine investigate his political rivals as the administration delayed military aid. Bolton's surprise announcement changed the political calculus for McConnell's no-witness strategy and appeared to increase the likelihood of additional testimony that could embarrass the president.

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

The announcement was a major boost for congressional Democrats, who have delayed transmission of the articles of impeachment for more than two weeks while seeking guarantees about the scope of the trial, including witnesses. Democrats have pressed for testimony from Bolton, as well as acting White House chief of staff Mick Mulvaney and several other of the president's men.

At least one Senate Republican, Mitt Romney of Utah, agreed Monday that it was imperative that Bolton testify, while Democrats insisted that Republicans' refusal to allow him to tell his story would be tantamount to a "coverup."

Bolton has firsthand knowledge of internal White House deliberations, and according to testimony in the House probe, reacted angrily to Trump's pressure on Ukraine to investigate former vice president Joe Biden, now a 2020 candidate, and a debunked theory about Ukraine interfering in the 2016 election.

The Bolton announcement came as lawmakers returned to Washington from a two-week congressional recess, with leaders still at loggerheads over the parameters of the impeachment trial.

House Speaker Nancy Pelosi, D-Calif., has been holding the two charges - abuse of power and obstruction of Congress - in hopes of strengthening Senate Minority Leader Chuck Schumer's hand in negotiating with McConnell, R-Ky., for witnesses. Bolton's statement seemed to create a shift in leverage in Democrats' favor.

"Will the United States Senate conduct a fair impeachment trial of the president of the United States? Will we search for all of the facts, or will we look for a coverup, a sham trial, on one of the most important powers the Founding Fathers gave the American people?" Schumer, D-N.Y., said on the Senate floor.

McConnell, for his part, made clear that once the articles are transmitted to the Senate, he will move ahead on the first phase of the trial - opening arguments - without a decision on witnesses.

"That was good enough for President Clinton, so it ought to be good enough for President Trump," McConnell said in remarks on the Senate floor, citing the impeachment trial 21 years ago. "Fair is fair."

Two moderate Senate Republicans - Susan Collins of Maine and Lisa Murkowski of Alaska - have said they're open to hearing from witnesses. And Romney threw his support behind calling Bolton, increasing the likelihood that moderates may force McConnell's hand eventually.

"I would like to be able to hear from John Bolton," the 2012 GOP presidential nominee told reporters. "What the process is to make that happen, I don't have an answer for you. The leaders are trying to negotiate that process right now. . . . What's important is that we hear from him."

Still, the same centrist Republican senators signaled that they were willing to start the trial without a deal for Bolton's testimony, keeping McConnell in firm control for now as he works to delay any decision on additional witnesses until after House Democrats present their case and the president's defense team rebuts it.

"There are a number of witnesses that may well be appropriate for the stage three, of which he would certainly be one," Collins said of Bolton.

When Bolton emerged as a potential witness in the fall, he indicated that he was willing to defy the White House and testify in the House impeachment inquiry - but only if issued a subpoena and if a federal court cleared the way. His statement Monday that he would participate in the proceedings without a court decision marked a shift in his position.

The Bolton development comes just days after McConnell had used the holiday break to persuade Trump to back a quick, no-witness trial. The president, known for his scorched-earth approach, had wanted to use the proceedings to go after Biden.

But McConnell, according to aides who were not authorized to speak publicly, explained that if Trump called his own witnesses, Democrats would insist on theirs. "Mutual assured destruction," he called it, warning against summoning anyone.

The major question for McConnell is whether he can hold the line - particularly after Bolton says he's ready to tell his story. A subpoena requires a simple majority of 51 votes, and Democrats would need just four Republicans to break ranks.

Bolton's testimony could be damaging to Trump politically, according to a person close to him who spoke on the condition of anonymity because they were not authorized to discuss private conversations. People familiar with Bolton's tenure at the National Security Council expect him to corroborate other impeachment witnesses' testimony that he was aghast that U.S. military aid was being held back as the president and his allies pressured Ukraine to open politically advantageous probes, according to people familiar with his views.

In recent months, Bolton has confided to friends that he was deeply troubled by his time at the White House and the president's behavior, but has declined to offer many details, the person said, adding that Bolton's support for Trump's hard line on Iran would not influence any possible testimony.

"Those are different issues. One doesn't affect the other," the person said.

Still, other Bolton associates have privately said that he wants a future in Republican politics and does not want to be seen as a turncoat on Trump or someone who is trying to ingratiate himself with the president's critics. They noted, for instance, that his statement Monday came from his political action committee's office as an example of how he's trying to build out his operation even as he deals with legal issues.

Additionally, people close to him note that Bolton also has an expansive view of presidential power. As a result, it is unclear whether he would testify that he believes Trump overstepped his constitutional authority in his dealings with Ukraine.

Democrats have sought Bolton's testimony for months, particularly after former National Security Council aides told impeachment investigators of his concerns regarding Trump's conduct toward Ukraine. Russian affairs director Fiona Hill testified that Bolton exploded in frustration after a White House meeting July 10 in which Gordon Sondland, the U.S. ambassador to the European Union, pressed Ukrainian officials to open investigations into Biden and the 2016 campaign.

Bolton, Hill said, equated the discussions to a "drug deal" - and told her to immediately report it to John Eisenberg, the top lawyer for the National Security Council.

Additionally, William Taylor, the then-acting ambassador to Ukraine, testified that Bolton was "very sympathetic" when he expressed concerns to the then-national security adviser that military aid to Ukraine was being leveraged for political favors. Bolton recommended that Taylor send a "first person" cable to Secretary of State Mike Pompeo about his worries to document the issue. And Taylor later heard from other State Department officials that Bolton was working to get the aid to Ukraine.

Senate Republicans appeared divided on the possibility of hearing from Bolton.

Sen. John Cornyn of Texas, who is close with McConnell, told Guy Benson, the political editor for conservative Townhall.com, that Bolton's testimony could be "helpful to the president" and that he'd like to know more about what he'd say. Other Republicans, including Sen. Marco Rubio of Florida, suggested that the Senate should only consider the evidence already gathered in the House, thereby precluding Bolton.

"I just don't think we're under any obligation to consider anything outside the record of what was created for the articles they sent us," Rubio told reporters.

Democratic officials last week predicted Pelosi would deliver the articles to the Senate as soon as this week, though the speaker has kept her plans close and her office has refused to detail her timeline.

Sen. Josh Hawley of Missouri and several other Republicans introduced a resolution Monday to dismiss the impeachment articles without a trial since Pelosi has yet to deliver the documents, though the measure appears unlikely to get a vote.