The Republican caucus is fracturing over how to respond to reports that the CIA believes the Russian government intervened in the American election to help Donald Trump — though an important Republican is now on board with looking into the matter.

Since the Washington Post’s bombshell story this weekend, at least five leading Republican senators — including John McCain and Lindsey Graham — have called for a congressional investigation into Russian hacking. Then on Monday, Majority Leader Mitch McConnell became the most powerful Republican to call for an investigation.

McConnell said he wanted a “bipartisan” congressional investigation into Russian interference, ruling out a special committee and instead saying the Senate Intelligence Committee could proceed with the probe.

“The Russians are not our friends,” McConnell said at a news conference, embracing a probe. “I hope those in responsibility in the new administration share my view.”

McConnell’s “hope” appears to be wrong: Trump has already adamantly declared the CIA report on Russian meddling a non-issue, dismissing it as a partisan attack.

But that’s why McConnell’s announcement is so important. It’s the strongest sign yet that the Republican caucus will be willing to go against the incoming Trump administration’s wishes over what some analysts argue is an unprecedented breach of America’s electoral system.

“This is a difficult position to be in: Partisans don’t like to investigate their own party’s president,” says Michele Swers, a congressional expert at Georgetown. “But now that you’re getting pushback from Republicans beyond McCain and Graham, it looks like Trump has something to worry about.”

How will the GOP break with Trump over Russia?

Over the weekend, the Washington Post and the New York Times disclosed that the CIA believes Russia hacked the Democratic National Committee to help Trump’s candidacy.

Writes Vox’s Timothy B. Lee: “In a ‘closed-door briefing on Capitol Hill last week,’ intelligence officials told senators that ‘it was now “quite clear” that electing Trump was Russia’s goal.’”

Since then, there’s been a flurry of activity among Capitol Hill lawmakers. Here’s a breakdown of what’s happened:

Democratic lawmakers moved swiftly to demand a full investigation of the CIA reports. Incoming Minority Leader Sen. Chuck Schumer sent out a strong statement that called for an immediate inquiry. “That any country could be meddling in our elections should shake both political parties to their core,” he said.

Trump then reacted by dismissing the CIA report as mere partisanship. "I think the Democrats are putting it out because they suffered one of the greatest defeats in the history of politics in this country," he said on Fox News the next day.

The GOP senators with a history of opposition to Russia then broke with the president-elect. Graham and McCain joined Schumer in releasing a statement calling an investigation essential.

“While protecting classified material, we have an obligation to inform the public about recent cyberattacks that have cut to the heart of our free society,” the joint statement said.

(McCain’s decision wasn’t just symbolic: He’s the chair of the Senate Armed Services Committee, which means he can call for an investigation and hearings with or without McConnell’s approval, according to Swers.)

Republican Sen. James Lankford, of Oklahoma, then also said that he hoped for an investigation into what had happened. Lankford is a member of the Senate Select Committee on Intelligence, and his announcement signaled that the appetite for Republican pushback extended beyond that of McCain and Graham, two longtime Trump critics.

The two most powerful Republicans in Congress, Speaker Paul Ryan and Majority Leader Mitch McConnell, have split on the Russia question thus far. Ryan has so far declined to say he wanted a congressional investigation, but that was before McConnell voiced his support for one early Monday morning. (Asked by reporters if he’d been in touch with Ryan about having the House play a role in the investigation, McConnell said yes, but didn’t indicate what Ryan’s response to calls for an investigation might be.)

The stakes here are larger than whether Congress looks into Russian hacking. As political scientist Seth Masket pointed out over the weekend, the key to bringing down Richard Nixon over Watergate was the resistance of recalcitrant Republican lawmakers:

Watergate came to an end when Republican members of Congress turned against Nixon. They are rightly viewed as national heroes today. — Seth Masket (@smotus) December 12, 2016

We still don’t know whether McCain or McConnell really will move forward with congressional inquiries, or if they’re merely reacting to an intense burst of pressure that will dissipate once the national media attention moves elsewhere.

But the past few days have opened the door to at least the possibility that they’re willing to buck their party’s leader over key questions of national security.