“I know he has a plurality of Republicans who have voted for him,” she added. “But I think in the course of this campaign, we are going to demonstrate he has no ideas. There’s no evidence he has any ideas about making America great, as he advertises. He seems to be particularly focused on making himself appear great. And as we go through this campaign, we’re going to be demonstrating the hollowness of his rhetoric.”

Mrs. Clinton also poked at Mr. Trump’s failure to release his tax returns. Told that Mark Cuban, the media executive and owner of the Dallas Mavericks, had expressed interest in being her running mate, Mrs. Clinton said she was “absolutely” open to considering business leaders, not just elected officials.

“Businesspeople, especially successful businesspeople, who are really successful — as opposed to pretend successful — I think, have a lot to offer,” said Mrs. Clinton, whose campaign has begun taunting Mr. Trump with a #PoorDonald hashtag on Twitter, suggesting that he is not nearly as wealthy as he claims. Mr. Trump has cited an audit by the Internal Revenue Service as his reason for keeping his tax returns private.

“We’ve got to get below the hype,” Mrs. Clinton said. “I think we’re beginning to find out, but I don’t think we know enough, and that’s why he should release his tax returns.”

In choosing a running mate, Mrs. Clinton said she would seek, above all, someone prepared to be president and “someone you can work with — someone you believe will be a good partner.”