The dam is beginning to crumble.

A new Washington Post-Schar School poll shows a startling shift in public sentiment in favor of the decision by House Democrats to open an impeachment inquiry into President Trump’s blatantly improper request that the Ukraine government help him dig up dirt on his leading presidential rival, former vice president Joe Biden.

The poll found nearly 6 in 10 of those surveyed support the investigation. About half of the public wants to see Trump removed from office over the “favor” he requested during that now-infamous July 25 telephone call with Ukrainian President Volodymyr Zelensky.

The impropriety is far clearer than the allegations of collusion and obstruction that fueled the investigation into Russia interference in the 2016 election, when large majorities said even considering impeachment would be an overreach.

The conversation with Zelensky occurred when the Trump administration was withholding $391 million in badly needed military aid from Ukraine. Americans know a shakedown when they see one.

What is surely most worrisome for Trump in the poll is its indication that his once-impregnable base could be giving way. It found that 28 percent of Republicans support the inquiry and 18 percent would like to see him removed from office.

Yes, those who back the idea of a congressional investigation are still a minority within the president’s party, but a rapidly growing one, their numbers up by 21 points since a Post-ABC poll from July, when the issue at hand was the findings of the special counsel’s Russia investigation. The revelation of Trump’s Ukraine call has shifted the dynamic across the political spectrum. The increase in support for an impeachment probe has been roughly the same among Democrats, Republicans and independents.

Even Trump’s supporters, it would appear, can see with their own eyes that his request of Zelensky was out of bounds — no matter how many times the president tells them the call was “perfect” — and that the whistleblower who reported it offered an accurate rendering of what transpired. A trove of text messages turned over by Kurt Volker, who resigned last month as Trump’s special envoy to Ukraine, have revealed that diplomats recognized in real time what was going on, irrespective of whether there was an explicit quid pro quo on the table.

No doubt, Republican misgivings have only been deepened by the president’s unhinged behavior since then. He throws around words like “treason” and warns of an impending civil war. He suggests that those who talked to the whistleblower deserve to be executed, and he publicly hints that trade negotiations with China might go more smoothly if that country also investigated Biden and his son Hunter.

There comes a time — and it might be arriving — when even Republicans can no longer dismiss these rantings as merely “Trump being Trump,” or, as Sen. Marco Rubio (R-Fla.) put it, “just needling the press, knowing that you guys were going to get outraged by it.” Could it be that the GOP base has finally grown exhausted with having to defend the indefensible? Or that impeachment might be the only brake on the impulses that have taken Trump further and further over the line?

Of course, this is only one poll, and no other has shown this kind of movement within the Republican base. But the overall trend line of support for the inquiry is confirmed by other surveys that have come out recently.

As sentiment builds in favor of what House Democrats are doing, the president is not helping his case — or his credibility — by standing in the way of lawmakers’ efforts to gather the facts from firsthand sources. The question is not if but when they will hear from people such as Gordon Sondland, the U.S. ambassador to the European Union and a central figure in the impeachment inquiry, whose deposition Tuesday was blocked by the administration.

Still, impeachment is a historic step, and Democrats must proceed cautiously and soberly.

Republicans are now demanding a vote on the House floor to authorize the inquiry to begin.

That is not something required by the Constitution or any congressional rule. But it is probably a concession that Speaker Nancy Pelosi (D-Calif.) should make. It would not increase the political exposure of her own members, given that 227 of them have already come out in favor of the inquiry.

And it would force Republicans to answer questions that many of them have been ducking: Do they condone Trump’s behavior? Do they really think there is nothing to see here?

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