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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.