Virginia Senator Tim Kaine has attended his first rally as Hillary Clinton’s vice presidential running mate, saying: “America was not built on fear”.
In remarks seen as attacking Republican presidential nominee Donald Trump, Mr Kaine also said the businessman had left “a trail of broken promises”.
The 58-year-old moderate Democrat could appeal to Republicans who have been unsettled by Mr Trump’s candidacy.
Mr Trump said Mrs Clinton and Mr Kaine “don’t look presidential to me”.
At the event in Florida, Mr Kaine recalled his time-fighting housing discrimination as a young lawyer, saying: “If you want to be right, be a pessimist, if you want to do right, be an optimist. I like to do right.”
Correspondents said Mr Kaine’s comments were pitched to appeal to more left-leaning Democrats, who may have supported Mrs Clinton’s one-time rival, Senator Bernie Sanders.
The Vermont senator had waged a strong campaign in the Democratic primary, championing liberal causes such as universal health care coverage.
The Trump campaign attacked Mrs Clinton’s choice of running mate, dubbing the senator from Virginia “Corrupt Kaine”.
As governor of the state, Mr Kaine accepted about $160,000 (£122,000) worth of gifts from political supporters, which was legal under the state’s lax gift laws.
“If you think Crooked Hillary and Corrupt Kaine are going to change anything in Washington, it’s just the opposite,” said Jason Miller, spokesman for the Trump campaign.
On Twitter, Mr Trump sought to drive a wedge between the Clinton campaign and voters who had supported Mr Sanders.
Several Republicans senators praised the choice of Mr Kaine, including Arizona Senator Jeff Flake.
“Trying to count the ways I hate @timkaine. Drawing a blank. Congrats to a good man and a good friend,” he wrote.
A former mayor as well as governor, Mr Kaine is an experienced politician and has been thoroughly vetted. He was on the short list to be President Barack Obama’s running mate in 2008.
His home state of Virginia is seen as a key battleground in the election. Vice-presidential candidates can typically give tickets a slight advantage in their home states.
Mr Kaine embraced the typical role of the running mate, attacking the rival nominee.
He called on Mr Trump to release his tax returns and criticised his business dealings, including his Atlantic City casinos and Trump University.
“Trump leaves a trail of wrecked lives everywhere he goes,” Mr Kaine said.
Mr Kaine also speaks fluent Spanish, which could help the Clinton campaign’s outreach with Latinos – a key voting bloc.
On Saturday, Mr Kaine frequently spoke to the crowd in Miami in Spanish.
“Bienvenidos a todos en nuestro pais. Porque somos Americanos todos. [Welcome everyone to our country, because we are all Americans],” Mr Kaine said to cheers.
However, left-wing Democrats have questioned his stances on banking reform and abortion rights.
A Catholic, Mr Kaine personally opposes abortion and has favoured restrictions on late-term or “partial birth” abortions.
Also, he was recently among 70 senators who signed a letter urging the Consumer Financial Protection Bureau not to enact regulations that could harm community banks and credit unions.
The Democratic Party is already pushing back, enlisting President Obama to vouch for Mr Kaine.
“Like Hillary, Tim is an optimist. But like Hillary, he is also a progressive fighter,” Mr Obama wrote.
The Democrats’ four-day convention starts on Monday in Philadelphia with speeches by First Lady Michelle Obama and Mr Sanders.