WASHINGTON — After all the ways that this year’s presidential election has made history, Donald J. Trump found a new line to cross on Wednesday, when he said at a news conference that Russia should hack his opponent, Hillary Clinton.

“Russia, if you’re listening, I hope you’re able to find the 30,000 emails that are missing,” Mr. Trump said, in reference to the private email server Mrs. Clinton used while she was secretary of state. “I think you will probably be rewarded mightily by our press.”

There is simply no precedent for this: A presidential candidate publicly appealing to a foreign adversary to intervene in the election on his behalf.