Monday was the day House Speaker Nancy Pelosi (D-Calif.) had to know might be coming but did her best to forestall. It was the day the dam she had erected against the Democrats’ impeachment fervor was breached.

Despite polls long showing about three-quarters of Democratic voters favor impeachment, Pelosi and her fellow leaders had done a good job keeping their party’s congressional contingent unified behind a more cautious approach. While a handful of mostly backbenchers have kept beating the impeachment drum, it hadn’t really filtered up into the ranks of top leaders and presidential candidates.

After the release of the Mueller report, that’s changing. Sen. Elizabeth Warren (D-Mass.) was the first big-name 2020 candidate to come out in favor of impeachment, and on Monday Sen. Kamala D. Harris (D-Calif.) joined her.

In some ways, it’s a wonder it’s taken this long. The 2020 Democratic field has already lurched to the left on a number of issues the party used to avoid for fear of looking too extreme: single-payer health care, jobs guarantees, marijuana legalization, free or debt-free college and reparations for slavery. For members like Warren and Harris, supporting impeachment while other 2020 Democrats remain reluctant is a great way to get to the left of your opponents. And there’s very little downside in the primaries, given this is a 75-25 issue.

But while the vast majority of Democratic voters have told pollsters they favor impeachment, there hasn’t really been a national movement. Part of that was because everyone was waiting to see the Mueller report, and part of that was that there really hasn’t been a national leader for the movement.

Neither of those reasons applies any more. The Democratic base feels righteously peeved about what they see in the Mueller report and in Attorney General William P. Barr’s questionable actions in releasing it. They now have a document with which they can make the case that President Trump committed crimes. Special counsel Robert S. Mueller III himself didn’t reach that conclusion but only because he didn’t view it as his place to do so. If you look closely at his report, there are four or five areas in which Mueller seems to believe there is substantial evidence Trump’s actions meet the criteria for obstruction of justice.

What’s also significant here is the way in which Warren and Harris are talking about impeachment. Warren’s words in particular seemed geared toward rebutting Pelosi’s argument for a more cautious approach.

A few weeks back, Pelosi set the threshold for impeaching Trump as getting bipartisan buy-in from Republicans. As I argued at the time, she was setting the bar almost impossibly high, given how unified the GOP base remains behind Trump. She was also effectively giving Republicans veto power over impeachment, which requires only a majority vote in the Democratic-controlled House. And she made clear the reason she was setting such a high bar was because it would be a politically arduous process.

“Impeachment is so divisive to the country that unless there’s something so compelling and overwhelming and bipartisan, I don’t think we should go down that path, because it divides the country,” Pelosi said. “And he’s just not worth it.”

As I also wrote at the time, there was a real question about whether the Democratic base and fellow Democratic members of Congress would embrace such a standard. Warren served noticed Monday that she simply doesn’t agree with Pelosi. In fact, she was asked about Pelosi’s political calculus, and she flatly rejected it.

ANDERSON COOPER: What do you say to those Democrats who say, look, this is not the time, it’s going to take away the focus from winning in 2020? Speaker Pelosi told her caucus again just today that she no plans to immediately initiate impeachment proceedings.

WARREN: So, there is no political inconvenience exception to the United States Constitution.

She continued: “This is not about politics. This is about principle. This is about what kind of a democracy we have.”

And: “If there are people in the House or the Senate who want to say that’s what a president can do when the president is being investigated for his own wrongdoings or when a foreign government attacks our country, then they should have to take that vote and live with it for the rest of their lives.”

Harris’s version was more sympathetic to Pelosi’s political thinking, but she also emphasized principle over pragmatism.

“I’ve not seen any evidence to suggest that [Senate Republicans] will weigh on the facts instead of on partisan adherence to being protective of this president, and that’s what concerns me and what will be the eventual outcome,” Harris said. “So we have to be realistic about what might be the end result. But that doesn’t mean the process should not take hold.”

The Democratic Party is currently engaged in a battle between its head and its heart — between a thirst for the power that has eluded it in recent years and a real sense that impeaching Trump is simply the right thing to do. Warren and Harris are now giving Democrats license to pursue the latter course — to make this a moral calculation rather than the political one Pelosi has argued in favor of.

If Democrats start joining their ranks and rejecting the pragmatic approach, Pelosi and her fellow leaders are going to be faced with a really difficult decision.