PRESIDENT TRUMP may be formally refusing to cooperate with a congressional impeachment inquiry, but on Thursday his acting chief of staff revealed all that House investigators need to know in order to determine whether Mr. Trump abused his oath of office.

Yes, Mick Mulvaney said, Mr. Trump withheld military aid from Ukraine, money that had been appropriated by Congress and was desperately needed to resist Russian aggression, in order to induce the government of Volodymyr Zelensky to investigate a conspiracy theory about the hacking of the Democratic National Committee (DNC) during the 2016 election. In other words, the president was using U.S. aid as leverage to advance his personal political agenda.

“We do that all the time with foreign policy,” Mr. Mulvaney said with stunning brazenness. “I have news for everybody: Get over it. There is going to be political influence in foreign policy.”

Hours later, apparently realizing (or having been told) that confession might not be the most astute defense strategy, he tried to walk back his remarks, saying “there was absolutely no quid pro quo.” But his earlier statement could not have been clearer.

The chief of staff’s declaration represented an about-face in the White House defense on the Ukraine affair. Until now, Mr. Trump’s main argument has been that the whistleblower who described his July 25 phone call with Mr. Zelensky and his subordination of U.S. diplomacy to his reelection campaign had gotten it all wrong: There was no quid pro quo. But testimony to Congress by half a dozen present and former State Department and White House officials, and a rough transcript of the phone call, have fully confirmed the whistleblower’s memo.

The new defense outlined by Mr. Mulvaney: Okay, we did it. So what?

The chief of staff argued that every administration seeks concessions from foreign governments in exchange for aid or meetings with the president. That’s true — but the difference here is that Mr. Trump was demanding help for his personal political cause, not for the United States. In the call with Mr. Zelensky, which took place a week after he suspended $391 million in military aid, Mr. Trump asked for an investigation of the far-fetched theory that the DNC server hacked by Russia had somehow been spirited away to Ukraine.

There is no evidence for that claim and thus no cause for an investigation. But Mr. Trump is seeking to undermine the unanimous conclusion by U.S. intelligence agencies and special counsel Robert S. Mueller III that Russia penetrated the server and subsequently leaked emails stored there to help Mr. Trump defeat Hillary Clinton.

During the same phone call, Mr. Trump asked for an investigation of Democratic presidential candidate Joe Biden and his son Hunter, who served on the board of a Ukrainian gas company. Mr. Mulvaney tried to draw a distinction between seeking a probe of the 2016 election and one of Mr. Biden, which he claimed was not linked to the aid. But he confirmed that the president had told his aides to arrange his dealings with Ukraine through his personal lawyer, Rudolph W. Giuliani. As testimony and documents provided to Congress clearly show, Mr. Giuliani subsequently demanded that Mr. Zelensky issue a statement promising to investigate the Biden-associated gas company, as well as the 2016 election.

There is still more to learn about the Ukraine affair. But this much is now obvious: Mr. Trump conditioned U.S. defense aid, as well as a visit to the White House, on the Ukrainian president’s help in providing him with political dirt.

It was a quid pro quo. It was corrupt. And his chief of staff confessed it, in the cynical expectation that Republicans would not hold Mr. Trump accountable. If Mr. Mulvaney is right, our political system will be grievously damaged.

Read more: