Even Donald Trump might agree that a major reason he won the 2016 election is because voters couldn’t abide Hillary Clinton’s legacy of scandal, deception and stonewalling. Yet on the story of Russia’s meddling in the 2016 election, Mr. Trump and his family are repeating the mistakes that doomed Mrs. Clinton.

That’s the lesson the Trumps should draw from the fiasco over Don Jr.’s June 2016 meeting with Russians peddling dirt on Mrs. Clinton. First Don Jr. let news of the meeting leak without getting ahead of it. Then the...