Senate Majority Leader Mitch McConnell began the new year by confidently issuing a thinly veiled threat to Nancy Pelosi, his Democratic counterpart in the House.

If Pelosi didn’t want to immediately deliver the formal articles of impeachment to the Senate, a move that would launch the upper chamber’s constitutionally mandated impeachment trial, he was content to wait a few days before considering his options.

One of those options gaining steam among Senate Republicans is a precedent-setting change to Senate rules to allow a trial to begin or dismiss it without the House’s involvement.

“We can’t hold a trial without the articles. The Senate’s own rules don’t provide for that,” the GOP leader tweeted. “So, for now, we are content to continue the ordinary business of the Senate while House Democrats continue to flounder.”

Just hours later, however, former national security adviser John Bolton threw a wrench into McConnell’s plans for a game of constitutional chicken with House Democrats. Bolton, whose testimony Democrats desire, said he’d be happy to appear under oath – the president’s claims of executive privilege notwithstanding.

Bolton’s departure from the Trump administration three months ago was so tense that he and the president had a Twitter fight about the way Bolton left. Trump said he ousted him; Bolton claimed he left of his own accord.

The two men clashed repeatedly over foreign policy, and that acrimony apparently extended into the very heart of the impeachment charges involving aid to Ukraine. Former National Security Council staffer Fiona Hill testified that Bolton told her that he was “not part of whatever drug deal” State Department officials were “cooking up.”

Even after Bolton said Monday that he would voluntarily testify, McConnell shrugged off Democrats’ demands that a Senate trial include such witnesses. “We don’t create impeachments,” he had said on the Senate floor in December. “We judge them.” McConnell repeated as much Monday afternoon after dismissing Democrats’ condemnations of Trump’s decision late last week to kill Iran’s top military with a drone strike.

“At this dangerous time, House Democrats continue to play political games with their partisan impeachment of the commander-in-chief,” McConnell said, adding that the House Democrats' hearings were “the most unfair, most rushed impeachment inquiry in history.”

Because Republicans hold a slim Senate majority, overruling McConnell on barring witnesses from testifying in the trial would take the defections of four GOP senators. Before Bolton came forward Monday, that seemed unlikely. Afterward, Democrats expressed hopes that McConnell was in a greater bind.

Retweeting an NBC News alert about Bolton’s willingness to testify, Pelosi argued, “The President & Sen. McConnell have run out of excuses.”

Rep. Adam Schiff, who chairs the House Intelligence Committee and spearheaded the House impeachment hearings, quickly chimed in as well, calling Bolton “an important witness” who was so concerned about “misconduct involving Ukraine” that he called it a “drug deal.”

“Bolton refused to testify in the House, following Trump’s orders,” Schiff tweeted. “Now he is willing to come forward. The Senate must allow testimony from him, Mulvaney and others. The coverup must end.”

Bolton’s willingness to tell his side of the story may alter the impeachment dynamic. While the Senate could decide to narrowly limit his testimony, the House, which never subpoenaed Bolton, could decide to take up this opportunity to do so, further delaying the timing of sending impeachment articles to the Senate. At the same time, the mini furor over Bolton drew attention to the rushed and partisan nature of the process in the first place.

Jonathan Turley, the the George Washington University constitutional law professor who served as a star GOP witness during the House impeachment hearings last month, said Bolton was simply reaffirming his willingness to testify after Democrats never “bothered to send him a subpoena let alone try to enforce it.”

“This is precisely why the rush to impeachment was such a historic blunder,” he said.

Democrats disputed Turley’s assertion, arguing that Bolton had previously withheld a decision on whether he planned to testify until the outcome of a court case brought by his former deputy, Charles Kupperman. House Democrats had subpoenaed Kupperman, but the White House ordered him not to comply.

The House has since withdrawn the subpoena, and a federal judge last week dismissed Kupperman’s lawsuit challenging it because he said there was no expectation that the House would reissue it.

Bolton appeared to be responding to that decision, stating Monday that “it now falls to the Senate to fulfill its constitutional obligation to try impeachments, and it does not appear possible that a final judicial resolution of the still-unanswered constitutional questions can be obtained before the Senate acts.”

McConnell still has the power to shut down the impeachment process entirely, taking the political pressure off a handful of centrist Republicans, including Sen. Susan Collins of Maine, who is facing the reelection fight of her life. The Kentucky Republican could simply decide to dismiss the impeachment and argue that Pelosi undermined her own rationale for rushing the impeachment process by withholding the articles in an attempt to gain leverage over how the Senate conducts its trial.

“Faced with the House manipulating the system, the Senate can change its rules and simply give the House a date for trial, then declare a default or summary acquittal if House managers do not come,” Turley wrote in a piece in The Hill.

Missouri GOP Sen. Josh Hawley, who has emerged as a major Trump ally, introduced a measure to do just that on Monday, and 11 other Senate Republicans signed onto it, including Sens. Joni Ernst of Iowa and John Barrasso of Wyoming, who are members of party leadership.

The resolution would change Senate rules to allow a vote on the articles of impeachment within 25 days of an impeachment vote, thereby undercutting the ability of House leaders to control the clock.

“If the Constitution is going to remain in effect, if the Senate is going to have the power, as the Constitution provides, to try cases, if the president is going to get his day in court, if the American people are going to have the ability to have this issue resolved, to see the facts, to get a verdict, it has to act,” Hawley said on the Senate floor Monday.

McConnell was noncommittal about escalating tensions by using such a bold move, stating that he is happy “for now” for the issue to remain in a state of limbo.

Senate experts warn that such a change to chamber rules could devolve into a decision by the next Senate, possibly under Democratic control, to go nuclear on the legislative filibuster, a move that maintains broad bipartisan opposition.

Yet, over the weekend, bigger cracks started to form in that GOP front when Lindsey Graham, who as Senate Judiciary Committee chairman would lead the impeachment trial, said he is open to a rules change that would allow the chamber to move forward without receiving the articles from Pelosi.

If Pelosi doesn’t deliver the articles in one week, Graham told Fox News the Senate should “take matters into our own hands.”

Republicans, he added, should only continue to wait "days, not weeks."