One of the House Democratic leadership’s principal arguments against impeaching Donald Trump was that polls, for the most part, showed Americans against the idea. For House Speaker Nancy Pelosi (D-Calif.), this was no small detail: she considered public support a prerequisite to a legitimate process.

And yet, for months, no national poll showed proponents of presidential impeachment outnumbering opponents. Yesterday, that changed.

Americans are split, 49%-46%, on whether they approve of Democrats’ impeachment inquiry into President Trump, and independents at this point are not on board, a new NPR/PBS NewsHour/Marist Poll finds. […] The poll was conducted Wednesday night with live phone interviewers. That was one day after House Speaker Nancy Pelosi announced the impeachment inquiry, but before a whistleblower complaint about the president’s call with the Ukrainian leader was released to the public.

The full results are online here (pdf). Looking through the crosstabs, the divisions are largely in line with expectations, with traditional Republican constituencies opposing impeachment and traditional Democratic constituencies supporting it.

But of particular interest was the shift over time: in April, the same NPR/PBS NewsHour/Marist poll asked about Trump’s impeachment and found that opponents easily outnumbered supporters, 53% to 39%. The numbers obviously haven’t flipped, exactly, but now a plurality supports the idea.

This coincides with the latest results from some online pollsters, Morning Consult and YouGov, which released similar findings yesterday afternoon.

It’s worth appreciating the likely explanations for the apparent shifts in public attitudes.

Broadly speaking there are two schools of thought on this. The first is that the shift among Democratic officials has helped change many voters’ minds. When the party was divided, many of the party’s voters followed suit, waiting for some kind of Democratic consensus to take shape. Once that happened this week, the argument goes, elements of the public responded in kind.

And while there may be some truth to that, I’m even more persuaded by the other explanation: public attitudes started changing when the circumstances changed. This isn’t a political landscape in which Americans reconsidered the Russia scandal and Robert Mueller’s findings, and drew new conclusions; it’s one in which a devastating new scandal emerged featuring a president who was caught trying to get a foreign government to help his re-election campaign.

To state the painfully obvious, the latest data is a brief snapshot in time, and attitudes on Trump’s impeachment will invariably change. Stuart Rothenberg warned yesterday, “Polling in the middle of a political hurricane is ridiculous,” and he has a point. Events are unfolding quickly, with new revelations coming to the fore, seemingly every few hours.

That said, Democratic lawmakers care how the electorate feels about this process, and the stronger the support for presidential impeachment, the stronger Democratic spines will be.