Democratic presidential candidate Hillary Clinton and Virginia Senator Tim Kaine campaigned together for the first time Saturday, sounding an optimistic message of hope in the future, in contrast to the past week's Republican Party convention, where many speakers warned the United States is in decline.
Clinton introduced Kaine to a crowd of cheering supporters in the state of Florida, saying she and her choice for vice president will "offer a very different vision" for America, one that calls for "building bridges, not walls" - a jab at Republican Donald Trump's call for building a wall between the U.S. and Mexico to keep out illegal immigrants.
"Senator Tim Kaine is everything that Donald Trump and Mike Pence [the Republican running mate] are not," Clinton declared. "He is ready to step into this job and lead on day one. And he is a progressive who likes to get things done."
Trump had no campaign appearances Saturday but spoke out in a series of tweets that appeared to be aimed at dividing Democratic voters, especially over economic issues. The Republican candidate labeled Kaine a "job killer" for his support of the Trans Pacific Partnership trade deal. The TPP was strongly opposed by Clinton's opponent during party primary elections and caucuses, Senator Bernie Sanders.
"Bernie supporters are outraged," Trump claimed.
President Barack Obama warmly welcomed Kaine to the Democratic ticket for November's general election. The senator from Virginia is "a true progressive" who "will make a great vice president," Obama said in a fund-raising appeal sent out early Saturday. "You just can't find anyone with a bad thing to say about him."
Clinton said the Democratic Party convention beginning Monday in Philadelphia will emphasize that "embracing diversity" makes America great, and she portrayed Kaine as a fighter for minorities and the impoverished.
She and Kaine spoke to a huge crowd in Miami, at Florida International University. Many of those present were Hispanic, and Kaine displayed his fluent Spanish, acquired when he worked in Honduras during a break from law school.
Standing beside Clinton, Kaine told the crowd, "We're going to be 'compañeros de alma' in this great 'lucha' ahead'' - "soul mates in this great fight ahead.''
Kaine introduced himself to the Florida voters, speaking about his family, his values growing up and his career as a mayor, governor and U.S. senator. Known as a staunch Roman Catholic, he said his family embraces "faith, family and work" as its main values.
Trump, meanwhile, sent a text message to his supporters declaring that Obama, Clinton and Kaine were "the ultimate insiders" - members of the Washington establishment that many American voters say they strongly oppose.
Trump implored his supporters not to "let Obama have a 3rd term" - another of his campaign themes depicting Clinton's policies as indistinguishable from those of the two-term president, who will leave office in January.
Kaine told Democrats Saturday that Trump is a callous businessman who "leaves a trail of wrecked lives wherever he goes."
Clinton said her running mate supports tighter restrictions on guns and equal rights for women, including the right to make their own decisions about abortion. She also hailed his record in support of equal rights for gay and transgender people, as well as immigration reform.
WATCH: Clinton, Kaine Promote Message of Optimism in Florida
"I believe in making a positive difference in people's lives," Kaine said, describing his experiences as a young man working in Honduras. While in that Central American nation, Kaine said, "I got a firsthand look at ... dictatorship. A few people had all the power and everybody else got left behind."
If elected in November, a Clinton-Kaine administration will aim to present during its first 100 days in office a new plan for immigration to the United States, with a specific program offering citizenship to arrivals from abroad.
Kaine also promised to stem the nation's gun violence epidemic, recalling his experience as Virginia governor during the mass shooting at Virginia Tech University in 2007 in which 32 people died - "the worst day of my life." He pledged that "Hillary and I will not rest" until the U.S. establishes a system of universal background checks and closes loopholes that allow criminals and terrorists to gain access to firearms.
"These are tough times for many people in our country, but we're tough people," Kaine said as he concluded his speech. "Tough times don't last, but tough people do."
Could appeal to moderates turned off by Trump
Kaine is a veteran Democratic politician who describes himself as "boring." He is a moderate who has the potential to attract voters repelled by Donald Trump and those who may have a hard time fully embracing Clinton.
"He's never lost an election. He was a world-class mayor, governor, and senator and is one of the most highly-respected senators I know," Clinton told CBS News.
"The most important consideration is his ability to step in as president, and he clearly has the experience, knowledge, intelligence and temperament to do that," said Jocelyn Bucaro, a convention delegate from Ohio who supports Clinton.
Kaine was born in Minnesota 58 years ago and is a lawyer trained at Harvard University, the same school that President Barack Obama attended. He took a year's sabbatical while in law school to work as a Catholic missionary and teacher in Honduras.
According to his biography, Kaine observed poverty up close in Central America and saw what it can do to the human spirit. His time there is said to have helped form his support for citizenship for undocumented immigrants in the United States — a stance likely to attract Latino voters.
Ben Monterroso, executive director of Mi Familia Vota, said Clinton "has chosen a running mate that has a track record of advocating and fighting for the issues that affect the Latino community and our nation: immigration, health care, women's rights and the environment."
Pick may be let down for liberals
Kaine's vice presidential candidacy may disappoint some progressive Democrats, some of whom had hoped that Senator Bernie Sanders' leftist policy proposals, which attracted wide support during the primary election campaign, might prompt Clinton to choose a vice-presidential candidate known more widely as a liberal.
When he was mayor of Richmond, Virginia's capital, Kaine supported a program called Project Exile, which tried to reduce violence by making illegal weapons possession a federal crime punishable by up to five years in prison. Some activists who feel that now-defunct program's methods were excessive have said the senator's record could cost him some support from African-American voters.
Nicole Lee, a civil-rights lawyer, said, "These measures were not used against white kids in the suburbs with guns. They were used against black kids in the cities."