It turns out President Trump can push his fellow Republicans too far. Senate Republicans stuck up for themselves, and their institution, on Tuesday by joining unanimously with their Democratic colleagues to call on the president to stop stonewalling. They asked him to release to the relevant congressional committees the complaint from a whistle-blower that an inspector general had said raised an “urgent concern about the president’s behavior.

On the need for greater transparency from this White House, lawmakers from both parties are in unusual agreement, at least for now. And the White House showed signs of backing down, signaling not that it would release the full complaint but that it might not block the whistle-blower from testifying.

The rare display of institutional solidarity in defense of American democracy may prove ephemeral. On the other hand, it can be hard to recognize turning points in the moment, and this week Mr. Trump’s outrages seemed to be stirring lawmakers from their state of political rigidity and passivity.

On Tuesday, the House speaker, Nancy Pelosi, announced she would open an impeachment inquiry. But when reports surfaced this spring that Rudy Giuliani, Mr. Trump’s personal lawyer, had been pushing Ukrainian officials to pursue a corruption investigation of former Vice President Joe Biden, the front-runner for the Democratic presidential nomination, there was no mass outrage, no collective gasp of horror.

But over the past couple of months, and more intensely the past couple of weeks, has come an accelerating accretion of more, and more alarming, information: a whistle-blower complaint had been filed with the inspector general of the intelligence community accusing Mr. Trump of, among other acts, pressing the new Ukrainian president, Volodymyr Zelensky, about Mr. Biden; the Department of Justice was blocking the inspector general from passing along the complaint, contra federal law; just days before speaking with Mr. Zelensky, Mr. Trump had directed the White House staff to withhold close to $400 million in military aid from Ukraine.

This episode of extreme politicking by Mr. Trump seems to go straight to questions of national security, and Democratic lawmakers who had been hesitant to call for impeachment began suggesting that it might be inevitable. On Sunday, Ms. Pelosi, a devout impeachment skeptic, gave the administration until Thursday to hand over the whistle-blower complaint or face “a whole new stage of investigation.” Senator Mitt Romney, Republican of Utah, felt moved to tweet that it was “critical” for the facts to come out.

Come Monday, and rolling into Tuesday, Washington was buzzing with a nervous energy. Everyone was on high alert, frantically scanning for signs of where things were headed next. Ms. Pelosi was canvassing her members about impeachment. With every House Democrat who stepped forward to speak about Ukraine — Debbie Dingell, Rosa DeLauro, John Lewis — the scramble to analyze the odds of impeachment began anew. In a Monday op-ed in The Washington Post, seven freshman House Democrats, including some from districts Mr. Trump won in 2016, came out in favor of a formal impeachment investigation. Twitter was awash in clichéd metaphors describing the shifting politics — the dam was breaking, the tide was turning, the winds were shifting. (In the real world, it always bears remembering, most people were less transfixed by the news from Washington.)

Republicans remained notably tepid about rushing to the president’s defense. The Senate Intelligence Committee, led by Richard Burr of North Carolina, opened an investigation into the matter and made clear it wanted to hear from the whistle-blower as soon as possible. Mitch McConnell, the Senate majority leader, publicly asserted that he had pushed for the funding for Ukraine and received no explanation for why it had been held up by the administration.

At some point Tuesday, the rumbling began that Ms. Pelosi would hold an afternoon news conference to announce the opening of a formal impeachment inquiry. Shortly after 2 o’clock, Mr. Trump tweeted that he had authorized the release of the “complete, fully declassified and unredacted transcript” of his call with Mr. Zelensky.

If the president was hoping this would ease the rising pressure, he was mistaken.

Democrats are not content to receive a transcript provided by the administration. Nor should they be. The allegations at hand are complicated and serious and call for the whistle-blower complaint in its entirety to be handed over to Congress. (The complaint is said to be about multiple concerning acts.) Ms. Pelosi conveyed this to the president Tuesday morning.

At 5 o’clock, Ms. Pelosi went before the nation and, in a five-minute statement, laid out the basic concerns about the president’s behavior, including his attempt to prevent Congress from learning about that behavior. “The president must be held accountable,” said Ms. Pelosi. “No one is above the law.”

There will be no more push and pull among Democrats about whether to hold an official impeachment inquiry. With apologies to Twitter, the trigger has been pulled, the Rubicon crossed, the die cast.

After months — years even — of watching Mr. Trump behave as though he answered to no one, many lawmakers seemed almost relieved that the showdown had arrived. Now that it has, lawmakers of both parties must proceed with care. Rarely have the stakes been so high.

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