MIAMI — Hillary Clinton debuted her running mate, Senator Tim Kaine of Virginia, to boisterous and bilingual cheers here on Saturday, calling him a “progressive who likes to get things done” even as some liberal Democrats began making clear that they were disappointed with her choice.

“I have to say, Senator Tim Kaine is everything that Donald Trump and Mike Pence are not,” Mrs. Clinton said, drawing a quick contrast with the Republican ticket as she introduced her own No. 2 to the nation.

Appearing comfortable in his new role as Mrs. Clinton’s top cheerleader and weapon against Mr. Trump, Mr. Kaine bounded up to the microphone, then slipped easily between English and Spanish as he animated the mostly Latino audience by mixing political talk with homey reflections on his life.