By October 20, Trump’s approval rating had tumbled all the way to … 41.5 percent.

That’s not a huge surprise—the number hasn’t budged more than about a point and a half since spring 2017—and from that angle, it was just another week in the strange stasis of Trump’s America, where nothing works, but no one’s mind ever changes. If even such a catastrophic seven-day stretch is only good for a 0.7 slide in approval, surely nothing can change until at least Election Day 2020.

But there’s another story: Support for impeachment has gone up sharply over a few weeks, settling in the low-to-mid-50s in the past few days. This suggests that, even though the approval and disapproval ratings are barely changing, something important is happening.

Trump’s disapproval numbers, like his approval, have been uncannily stable for years. The people who approve of Trump are largely monolithic: They will support Trump no matter what. The disapprovers are more varied. Some of them are as fervently anti-Trump as the approvers are pro-Trump, and they have long wanted him removed from office. But some of them have disliked what Trump was doing, but still didn’t favor an impeachment inquiry or removing him from office.

Something has changed for this group over the past few weeks, and 10 to 15 percent of them have decided they back impeachment after all. Their precise reason is hard to discern. Was it the Ukraine-call transcript? The whistle-blower complaint? Foreign policy? The launch of an inquiry? Or simple fatigue with the endless parade of bad news from the White House? Some of them have multiple or varying reasons.

As Philip Bump of The Washington Post noted last week, Gallup has only once found support for impeachment as high as it was last week—and that was days before Richard Nixon resigned. But as Nixon’s resignation approached, his approval rating sank; Trump’s has remained stable, which is one reason there’s been no sign of his resignation anytime soon. He still hasn’t lost his base.

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But with impeachment looming, members of Trump’s base aren’t the people whose sentiments the president needs to track most closely. Should the House impeach him, it’s senators who will vote on whether to remove him. Republicans have the edge in the chamber, and none of them—not even Mitt Romney—has signaled they’d vote to remove Trump, much less enough GOP senators to get to the two-thirds required to convict.

In a set of new polls last week, Morning Consult found that five of the most vulnerable Republican senators up for reelection in 2020 have seen their approval slide over the past few months. They’re all doing better than Trump in their respective states, too—a sign that he is a drag on them. The reality is that few Republican senators have ever been enthusiastic about Trump. As long as it’s good politically for them to stick with him, they may do so. But if it proves politically advantageous to break with him, why wouldn’t they? (Question their morals, but not their math.)

This is still an extreme scenario at the moment, but it’s the weakness in Trump’s impeachment firewall, and it’s big enough that Fox News’s Chris Wallace reported on Sunday that one top Republican sees the chance of Trump being removed at one in five. Maybe nothing can make Trump’s base abandon him. But maybe that doesn’t matter.