There is nothing new about intelligence services for the United States and Russia hacking each other’s computer systems or trying to covertly influence other countries’ domestic politics. In recent years, Mr. Putin has invaded Ukraine, stoking political tensions there and in the Baltic States, and financed right-wing politicians across Europe; during the Cold War, the United States secretly tried to influence elections in Indonesia, Italy, Chile and Poland. What seems new here is the idea that a government would publicly disclose information it got from hacking into the computers of an American political party in order to influence an election.

The Democrats are eager to exploit a Trump-Putin connection. The notion that a foreign power, especially a newly aggressive Russia, might be trying to sway the election is antithetical to democratic principles and America’s interests. As secretary of state, Hillary Clinton, who became the Democratic nominee on Tuesday, tried to create a better working relationship with Russia, on issues like arms control and Iran, but the effort ultimately failed. Mr. Putin could also be retaliating against Mrs. Clinton for real or imagined perfidies; he accused her of meddling in Russia’s 2011 parliamentary election.