The former secretary of state will be formally nominated as the party's presidential candidate for the Nov. 8 election at next week's Democratic convention in Philadelphia. She leads Trump in many opinion polls.

Clinton's choice of a running mate could give her campaign momentum heading into the convention, as the fight for the White House begins a more than three-month push to the finish.

Clinton acknowledged in an interview earlier this week that even Kaine admits he is boring, and said she did not mind.

"I love that about him," she told Charlie Rose of CBS News and PBS. "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."

Kaine's first appearance with Clinton will be on Saturday at an event in Miami, a campaign aide said.