“Heard from White House — Assuming President Z convinces trump he will investigate/‘get to the bottom of what happened’ in 2016, we will nail down date for visit to Washington.”

So wrote Kurt Volker, until recently President Trump’s special envoy to Ukraine, to Andrey Yermak, an adviser to Ukraine’s president, Volodymyr Zelensky, on the morning of July 25, the same day Mr. Trump asked Mr. Zelensky for “a favor” in their now-infamous phone call. Among a trove of encrypted texts that Mr. Volker turned over to Congress last week, this message is brief and informal. It is also a textbook example of a quid pro quo, one intended to advance not American foreign policy goals but Donald Trump’s re-election.

As the Ukraine scandal continues to unfold, there has been much disagreement about whether Mr. Trump withheld nearly $400 million in military aid to the former Soviet republic in an attempt to bully its government into pursuing investigations that would benefit him politically. Mr. Trump had demanded that Ukraine look into whether former Vice President Joe Biden misused his office to protect his son Hunter, who served on the board of a Ukrainian gas company. (There has been no evidence to suggest this.) He also wanted an inquiry into the (debunked) conspiracy theory that Ukraine, not Russia, was behind the hacking of the Democratic National Committee in 2016. Democrats say Mr. Trump’s extortion attempt is undeniable, and even a few Republican lawmakers have expressed discomfort. But most members of the president’s party have stuck by him, insisting that a direct connection is not clear.

But Mr. Volker’s offer of July 25 could not have been more clear: If Mr. Zelensky agreed to give Mr. Trump the investigations he so desperately wanted, then Mr. Trump would give Mr. Zelensky the White House visit he so needed as a sign of the United States’ continued support of Ukraine in its conflict with Russia. It is an exchange so explicit that even Mr. Trump’s fiercest apologists cannot wish it away.