JOHN BOLTON has not yet testified or spoken anywhere in public about the Ukraine affair, but his unpublished manuscript is exerting a gravitational pull on the Senate trial of President Trump. The former national security adviser is reported to have written that Mr. Trump directly connected his freeze on military aid to Ukraine with his demand that the country’s president launch politicized investigations, including of Democratic presidential candidate Joe Biden, the former vice president. The result is that some Republican senators who previously insisted that there was no evidence of such a quid pro quo have now retreated to a new line of defense: Maybe there was but, if so, there is nothing wrong with it.

The new response has the advantage of acknowledging the mounting evidence that Mr. Trump used congressionally appropriated aid to pressure Ukrainian President Volodymyr Zelensky to intervene in the 2020 election campaign. “We basically know what the facts are,” Sen. John Cornyn (R-Tex.) told Fox News on Tuesday. Yet Mr. Cornyn and other GOP senators are now arguing that the behavior is not an abuse of power, merely a routine presidential act. “Presidents always leverage foreign aid,” said Mr. Cornyn.

That contention is as dangerous as it is wrong. Presidents do occasionally wield U.S. assistance to advance foreign policy ends. But Mr. Trump was manifestly seeking a personal gain. An investigation of Mr. Biden was not a goal of U.S. foreign policy. There was no domestic probe of his actions and no evidence that he was guilty of wrongdoing. On the contrary, the proof that the then-vice president was pursuing official U.S. policy when he intervened in Ukraine is overwhelming.

Republicans are relying heavily on the arguments of one of Mr. Trump’s lawyers, Alan Dershowitz, a criminal-defense specialist who has been offering constitutional interpretations sharply at odds with those of constitutional scholars. On Wednesday, he made the extraordinary claim that a president who executed a quid pro quo for his own personal political gain could not be guilty of an impeachable offense; only action for pecuniary return could qualify. Contradicting the position he took when President Bill Clinton was on trial, Mr. Dershowitz said that presidents cannot be impeached unless they commit criminal acts.

The implications of this position are frightening. If Republicans acquit Mr. Trump on the basis of Mr. Dershowitz’s arguments, they will be saying that presidents are entitled to use their official powers to force foreign governments to investigate any U.S. citizen they choose to target — even if there is no evidence of wrongdoing. Mr. Trump could induce Russia or Saudi Arabia or China to spy on Mr. Biden, or on any other of the many people subject to his offensive tweets. In exchange for any embarrassing information, the president might offer official favors, such as arms sales or a trade deal or the lifting of sanctions. Do Republicans really wish to ratify such presidential authority? Will they not object if the next Democratic president resorts to it?

Republicans are finally beginning to accept the facts of what Mr. Trump did — though all the facts will not be known unless they allow Mr. Bolton and other witnesses to testify. They must now draw the necessary conclusion from those facts: that what Mr. Trump did was wrong. After doing so, they could argue that the offense does not merit impeachment, or that any sanction should be delivered by voters. But a conclusion that the president did nothing wrong would inflict grave damage on our political system.

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