By Amanda Becker and John Whitesides

MIAMI/WASHINGTON (Reuters) - Democrat Tim Kaine made his first appearance on the campaign trail as Hillary Clinton's vice presidential running mate on Saturday, touting an optimistic view of America and leaping to attack Republican presidential nominee Donald Trump's record.

Joining Clinton at a rally in the battleground state of Florida, the bilingual Kaine peppered Spanish-language phrases into a speech focused on introducing himself to voters unfamiliar with the low-key U.S. senator from Virginia.

Kaine criticized Trump's recent suggestion he might not honor U.S. security commitments to NATO in Europe, and the real estate mogul's history of casino bankruptcies and founding the failed Trump University.

"When Donald Trump says he has your back, you better watch out," Kaine said, with Clinton sitting at his side nodding. "He leaves a trail of broken promises and wrecked lives wherever he goes. We can’t afford to let him do the same thing to our country."

In contrast, he said, Clinton "doesn't insult people, she listens to them. What a novel concept." He said they shared a common creed: "Do all the good you can."

Clinton unveiled her choice of Kaine late on Friday, grabbing the political spotlight from Trump, who accepted the Republican presidential nomination on Thursday night after a chaotic four-day convention.

The former secretary of state will formally be nominated as her party's presidential candidate in the Nov. 8 election at the Democratic convention in Philadelphia, which opens on Monday.

In choosing the soft-spoken Kaine, a former Richmond mayor and Virginia governor with a long establishment resume, Clinton opted for a steady and experienced hand who she hopes will offer a clear alternative to Trump's volatile campaign and his Republican vice presidential choice Mike Pence.

"Senator Tim Kaine is everything Donald Trump and Mike Pence are not. He is qualified to step into this job and lead on day one," Clinton told the Miami crowd.

Trump was unimpressed, saying on Twitter he had watched the joint appearance and "ISIS and our other enemies are drooling. They don't look presidential to me!"

In his speech, Kaine said he was an optimist and described his childhood in Kansas City helping his father in his metal-working shop and his Catholic mission to Honduras, where he helped teenagers with carpentry and welding and they taught him Spanish.

FAITH, FAMILY AND WORK

He said in Honduras he learned the values "Fe, familia, y trabajo" -- faith, family, and work.

Kaine became emotional when he recalled the 2007 shooting deaths of 32 people at Virginia Tech University during his stint as governor, calling it the worst day of his life. He promised to take on the National Rifle Association and fight for "common sense" gun control.

Judith Sweeney, 58, from West Park, Florida, said she knew little about Kaine before attending the speech but was impressed by his resume and liked his civil rights work and school reform efforts. "Wonderful, very experienced, an innovator," she said.

Peter Daou, a former adviser to Clinton who now owns a media company, said Kaine's speech would turn around some of the doubters about the choice.

"The combination of his tone, his demeanor, his life story -it just said a lot about her capacity to choose the right person and not listen to pressure from the outside," he said.

Clinton is hoping Kaine will help her appeal to independents and moderates, but some supporters of Clinton's Democratic primary rival, U.S. Senator Bernie Sanders of Vermont, were dismayed by the choice because of Kaine's past advocacy for an Asian free-trade pact.

But in a nod to party liberals, the Clinton campaign said Kaine will not support the final version of the 12-nation Trans Pacific Partnership, which is pending in the Senate. A spokeswoman for Kaine confirmed his opposition to the pact.

The decision drew cheers from liberal groups and labor leaders.

"We're glad to see the Clinton-Kaine ticket taking steps to campaign on big, bold, populist ideas that voters want to hear from Democrats," said Stephanie Taylor, co-founder of the Progessive Change Campaign Committee.

Richard Trumka, president of the AFL-CIO labor federation, said Kaine's decision to join Clinton in opposing the trade pact "sets an irretrievable, progressive path forward for America."

In picking Kaine, Clinton passed over liberal candidates who would have generated more grassroots enthusiasm like U.S. Senators Elizabeth Warren of Massachusetts and Cory Booker of New Jersey, and Hispanic Cabinet members Julian Castro and Thomas Perez.

Clinton tried to reassure party liberals, offering an extended list of Kaine's efforts on behalf of low-income workers, education and civil rights, and for expanded gun control laws and immigration reform.

"He fights for the people he represents, and he delivers real results," Clinton said, applying one of her favorite self-descriptions to him. "When I say he's a progressive who likes to get things done, I mean it."

Sanders supporters lost their bid to eliminate or reduce the influence of superdelegates, party leaders who are not bound to any candidate, during a meeting of the convention rules committee in Philadelphia. Sanders had complained about the superdelegates, which overwhelmingly backed Clinton.

But enough committee members backed the effort to let them take the fight to the convention floor during next week's convention, participants said.

(Editing by Alistair Bell)