At times, his language verged on apocalyptic. The Democratic Party, he said, is “fighting for no less than the survival of our republic,” and doing so against “a man that wants to turn back the hands of time, to send us back to the Dark Ages.”

In such a fight, he continued, “we must honestly ask ourselves whether those that we fight for can afford our gentleness.”

It is a message in keeping with the work that has made Mr. Avenatti a boldface name: his alliance with Ms. Clifford, who claims to have had an affair with Mr. Trump and is suing the president’s onetime fixer, Michael D. Cohen.

As Ms. Clifford’s lawyer, Mr. Avenatti has adopted the president’s brash manner and some of his tactics. He has a similar instinct for using the news media to his advantage; he seems always to be on one cable news show or another. His Twitter feed is sometimes combative, sometimes coy, virtually always provocative — an example of the tack he is now urging the Democratic Party to take.

There was a certain tension, however, in his speech, which mingled calls to arms with calls for Democrats to reach out compassionately to Trump voters whose support for the president may be wavering. Democrats should think of such voters “not as evildoers but as victims of a great con,” he said. “Decent people get conned all the time, and let’s face it, Trump is a very good con man.”