If the crowd was not entirely raucous throughout, Mr. Kaine seemed to reach a rhythm about 20 minutes into the speech, when he moved in and out of his Trump impression.

“Do you really believe him?” Mr. Kaine asked the crowd, before describing Mr. Trump’s failure to pay contractors. “Donald Trump’s whole career says you better not.”

Mr. Kaine went on.

“Our nation,” he said, “it is just too great to put it in the hands of a slick-talking, empty-promising, self-promoting one-man wrecking crew.”

As Mr. Kaine took the stage, Mr. Trump had made his own pitch to Sanders voters: His campaign sent an email to reporters titled, “Kaine Is Out of Step With Bernie Sanders’s Priorities,” noting the senator’s past support for trade pacts. While Mr. Kaine spoke, some delegates from California lodged a silent protest, holding signs opposing the Trans-Pacific Partnership trade deal.

Mostly, though, there were cheers.

Mr. Kaine flashed a penchant for call-and-response with his crowd: “Can I be honest with you about something?” he asked at one point.

“Can I tell you a funny thing about the Senate?” he said at another.

And he stirred the delegates by asking pointedly what Mr. Trump was hiding in his tax returns.

Mr. Kaine was dealt a difficult hand on Wednesday evening, given the other speakers on the schedule. Vice President Joseph R. Biden Jr. spoke not long before him, holding forth with trademark folksiness and a poignant remembrance of his son Beau, who died last year of brain cancer. Former Mayor Michael R. Bloomberg of New York made the independent voter’s case against Mr. Trump, suggesting that the Republican nominee was neither sane nor competent.