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.
“Fe, familia y trabajo,” he told the crowd of more than 5,000 people at Florida International University, explaining that faith, family and work defined his life.
But as Mr. Kaine sought to flex his language skills to appeal to Hispanic voters, he and Mrs. Clinton were also trying to mollify a growing backlash from the left against his record of support for global trade deals, which many voters in Rust Belt states blame for the loss of American manufacturing jobs.
Mr. Kaine was one of 13 Senate Democrats to vote in support of giving President Obama “fast track” authority on the Trans-Pacific Partnership, the 12-nation pact that has become a lightning rod this election year. In a clear sign of concern that the issue could be damaging, aides to both Democrats signaled on Saturday that Mr. Kaine would soon publicly adopt Mrs. Clinton’s current position on the deal and say that he no longer supported it in its current form.
After praising the Trans-Pacific Partnership as secretary of state, Mrs. Clinton said during the Democratic primary race against Senator Bernie Sanders of Vermont that the pact did not meet her “high bar” on protecting American workers.
Progressive groups have offered mixed reviews of Mr. Kaine, with MoveOn.org Political Action saying it supports him in light of the “racist, bigoted” message coming from Mr. Trump.
Democracy for America, a liberal group that backed Mr. Sanders, agreed with the anti-Trump sentiment but said the selection of Mr. Kaine was not helpful to the progressive movement.
“Kaine’s support for fast-track authority for the job-killing Trans-Pacific Partnership and recent backing of bank deregulation will make our work more difficult,” said Charles Chamberlain, the executive director of Democracy for America.
Liberals looking for a sign from Mr. Sanders himself did not immediately get one.
Shortly after Mrs. Clinton offered Mr. Kaine a spot on the ticket, he called Mr. Sanders. But Mr. Sanders, who endorsed Mrs. Clinton only about two weeks ago, did not comment about her vice-presidential selection or mention it on Twitter on Saturday. A spokesman said Mr. Sanders would most likely discuss the pick in his interviews on Sunday talk shows.
Some supporters of Mr. Sanders on social media said the choice of Mr. Kaine was more evidence that their “Bernie or Bust” missions must continue. Others who were resigned to Mrs. Clinton, however, said they could live with her decision. “He is more than competent for the job,” said Steve Todd, a delegate for Mr. Sanders from Pennsylvania.
Larry Cohen, an adviser to the Sanders campaign and a former president of the Communications Workers of America, said of Mr. Kaine, “People are minimizing the range of this man by putting him in some box I would argue he doesn’t belong.”
Mrs. Clinton and Mr. Kaine sought to address the complaints head-on. Mrs. Clinton highlighted his work on fair housing issues and gun control in the aftermath of the Virginia Tech shooting that killed 32 people in 2007, when Mr. Kaine was the governor. “Make no mistake,” Mrs. Clinton said. “Behind that smile, Tim also has a backbone of steel. Just ask the N.R.A.”
The rollout of Mr. Kaine’s candidacy in Florida was no accident.
The state is a crucial battleground, and Mrs. Clinton is moving to press her advantage with the state’s Hispanic voters, many of whom have been turned off by Mr. Trump’s hard-line immigration policies.
Mr. Kaine, who took a year off from Harvard Law School to work with Jesuit missionaries in Honduras, wasted little time showing off his bilingualism, often thrusting his fist in the air and looking back and flashing a wide grin at Mrs. Clinton, who sat on a stool behind him.
“Bienvenidos!” he told those in the crowd, welcoming them in Spanish and declaring that he and Mrs. Clinton would be “compañeros de alma in this great lucha ahead,” Spanish (mostly) for “soul mates in the fight ahead.”
Mr. Kaine offered a folksy emotiveness that could ease concerns among Democrats that Mrs. Clinton appears too stilted and scripted at times. Mr. Kaine, who dislikes teleprompters, often veered off his prepared remarks, seeming elated to be under such a bright spotlight.
Mrs. Clinton seemed to feed off the energy her new running mate gave the crowd as he alternated between zingers about Mr. Trump and a bit of braggadocio.
“I’ve never lost an election,” Mr. Kaine said. “I’m not about to let that change.”
Discussing how he weathers attacks from the National Rifle Association and other groups, he said, “That’s just like an extra cup of coffee to me, folks.”
David Axelrod, a former chief strategist for Mr. Obama, criticized Mrs. Clinton’s delivery, writing on Twitter that “someone, at long last, needs to urge her to let the mic do the work!” But hepraised Mr. Kaine as “natural, engaging, connecting.”
In an implicit contrast with Mr. Trump’s rollout of Mr. Pence, when he dominated the microphone as his vice-presidential candidate stood silently for much of the time, Mrs. Clinton perched on a stool, nodding and cheering, behind Mr. Kaine as he spoke for about 40 minutes.
During Mr. Kaine’s speech, the Trump campaign sent out emails in rapid succession, citing his “job-crushing” record in Virginia, claiming he lacked any achievements as governor and deriding him as a career politician.
After a search that began in April, Mrs. Clinton’s advisers hoped Mr. Kaine could help her bridge the wide divide she faces with male voters, who favor Mr. Trump by 13 percentage points, according to the latest New York Times/CBS News poll. White men favor Mr. Trump by 26 percentage points.
Mr. Kaine’s positions on trade could also hurt the Democratic ticket in this regard. Part of Mr. Trump’s appeal has been his steadfast, if unspecific, promise to bring jobs back to economically depressed parts of the country, largely attributing the decline to the Trans-Pacific Partnership, which his own running mate also supported.
“Tim Kaine has been praising the Trans-Pacific Partnership and has been pushing hard to get it approved,” Mr. Trumpposted on Twitter on Saturday. “Job killer!”
Mr. Kaine, though, seemed to relish the fight, criticizing Mr. Trump’s business record and foreign policy positions. “We’ve seen again and again when Donald Trump says he has your back, you better watch out,” he said.