Democratic vice-presidential nominee Senator Tim Kaine speaks on the third night at the Democratic National Convention in Philadelphia, Pennsylvania, US, July 27, 2016. [Photo/Agencies]

"Our nation, it is just too great to put it in the hands of a slick-talking, empty-promising, self-promoting, one-man wrecking crew," Kaine said in a keynote speech to the Democratic Party convention in Philadelphia.

US Democratic presidential nominee Hillary Clinton chose Kaine of Virginia, as her running mate last week, opting for a partner at the Nov 8 election who has wide experience in public service to draw a contrast with Trump who has never been elected to office.

Peppering his speech with phrases in Spanish, the bilingual Kaine said he and Clinton were "companeros de alma," or soul mates, and led chants from the crowd of "Si, se puede." ("Yes, we can.")

Kaine said Trump, a businessman and reality TV show host, was fundamentally untrustworthy.

"He never tells you how he's going to do any of the things he says he's going to do. He just says, 'believe me.' So here's the question. Do you really believe him? Donald Trump's whole career says you better not," Kaine said.

Senior Democrats and former national security figures lined up in Philadelphia on Wednesday to describe Trump as unable to steer America through the dangerous waters of today's world.

The Republican on Wednesday gave his critics fresh fodder for attack with remarks about Russia that the Clinton campaign said posed a possible national security threat.

Grabbing the spotlight at a news conference in Miami, Trump urged Russia to find and release tens of thousands of emails that Clinton did not hand over to US officials as part of a probe into her use of a private email system while she was secretary of state. Clinton has said those emails were private.