And the facts, once again, were inconvenient for the argument. For one thing, Rudy Giuliani, Mr. Trump’s personal lawyer, has been openly bragging for months about his efforts to get Ukraine to investigate Mr. Biden and his son Hunter for alleged corruption. Now we know that what was being dangled in exchange for that investigation was a coveted visit to the White House for the new Ukrainian president, Volodymyr Zelensky, and nearly $400 million in military aid that Ukrainians desperately needed in order to defend themselves against Russian aggression.

The regular delivery of this aid has been authorized for years by bipartisan majorities in Congress. But Mr. Trump refused to pay up, Mr. Taylor and Mr. Kent testified, until Mr. Zelensky publicly announced what would be, in effect, a public plug for the president’s re-election campaign. As Mr. Taylor said, “Withholding security assistance in exchange for help with a domestic political campaign in the United States would be crazy.”

Over and over, the Republican members of the House Intelligence Committee attempted to cast the hearings as a sham, a Democratic plot to remove the president. Mr. Taylor and Mr. Kent declined to play along. They came across not as angry Democrats or Deep State conspirators, but as men who have devoted their lives to serving their country, and for whom defending Ukraine against Russian aggression is more important to the national interest than any partisan jockeying.

Pressed at one point by a Republican to agree that a particular offense was not impeachable, Mr. Kent flashed some steely contempt for such political gamesmanship. “I’m here as a fact witness to answer your questions,” he instructed the congressman, John Ratcliffe of Texas. “Your constitutional obligation is to consider the evidence before you.”

At another point, Mr. Taylor said he had been critical of the Obama administration’s reluctance to supply Ukraine with anti-tank missiles and other lethal defensive weapons in its fight with Russia, and that he was pleased when the Trump administration agreed to do so. Republicans who pressed him on this particular view — as though it supplied some sort of “gotcha” revelation — seemed taken aback that a witness so dangerous to Mr. Trump would also forthrightly and calmly criticize a Democrat. But it turns out there are independent- and fair-minded people still, even in Washington.

What clearly concerned both witnesses wasn’t simply the abuse of power by the president, but the harm it inflicted on Ukraine, a critical ally under constant assault by Russian forces. “Even as we sit here today, the Russians are attacking Ukrainian soldiers in their own country and have been for the last four years,” Mr. Taylor said. “I saw this on the front line last week; the day I was there a Ukrainian soldier was killed and four were wounded.”

Mr. Trump’s abandonment of the Ukrainians came out most chillingly in a new piece of evidence Mr. Taylor offered. Last week, he said, one of his staff members told him of a phone call the staffer overheard between Mr. Trump and his ambassador to the European Union, Gordon Sondland, which took place in a restaurant on July 26, the day after Mr. Trump told Mr. Zelensky to “do us a favor though” — by announcing investigations into both the Bidens and a debunked conspiracy theory about the Democratic National Committee email server in 2016.