Hillary Clinton has named US Senator Tim Kaine as her running mate, opting for an experienced governing partner who will help her present the Democratic ticket as a steady alternative to the unpredictable campaign of Republican presidential rival Donald Trump.
The selection of Kaine, a self-described "boring" Virginian with a reputation for low-key competence, could appeal to independents and moderates, but it quickly angered liberal groups that object to his advocacy for an Asian free-trade pact.
The Spanish-speaking former Virginia governor and Richmond mayor fit Clinton's long-stated criteria that the vice presidential choice be a capable and reliable partner who is ready to take over the presidency if necessary.
Clinton made the announcement via Twitter and a text message to supporters on Friday after the first day of a two-day campaign swing in Florida.
She called Kaine to tell him about 40 minutes before the announcement, and called President Barack Obama shortly after Kaine.
The former secretary of state will be formally nominated as the party's presidential candidate for the November 8 election at next week's Democratic convention in Philadelphia.
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.
A campaign official said Clinton was impressed with Kaine's down-to-earth style when she campaigned with him in Virginia last week.
Kaine's first appearance with Clinton will be on Saturday at an event in Miami, a campaign aide said.
Liberal groups, which had pressured Clinton not to pick Kaine because of his support for fast-track authority for the White House to negotiate the Trans-Pacific Partnership, were dismayed by the choice.
Critics of the Asia free-trade deal, including Trump and Clinton's Democratic primary rival Bernie Sanders, say it would be unfair to US workers and kill jobs. Clinton praised the deal when she was secretary of state, but has since distanced herself from it.
"Republicans will run hard against Democrats on trade this year. Unfortunately, since Tim Kaine voted to fast-track the Trans-Pacific Partnership, Republicans now have a new opening to attack Democrats on this economic populist issue," said Stephanie Taylor, co-founder of the Progressive Change Campaign Committee.
Hispanic activists also may be annoyed with the pick of Kaine given that Latino candidates were again passed over, though some Latino advocacy groups praised the choice.
"She 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," said Ben Monterroso, executive director of Mi Familia Vota.
Top Republicans were quick to criticise Clinton's choice.
The Trump campaign called Kaine "an ethically challenged insider" and called Clinton and Kaine a "Status Quo" ticket.
"If you think Crooked Hillary and Corrupt Kaine are going to change anything in Washington, it's just the opposite," campaign aide Jason Miller said in a statement.
Republican National Committee chairman Reince Priebus condemned the pick, saying Clinton spent the past week pandering to Sanders and grassroots Democrats, and now "has chosen someone who holds positions that she's spent the entire primary trying to get to the left of".
But Republican senator Jeff Flake of Arizona, who has been critical of Trump, said on Twitter that he was trying to count the ways he hated Kaine. "Drawing a blank. Congrats to a good man and a good friend," Flake said.
Kaine, a member of the Senate foreign relations and armed services committees, has been a leading voice calling for a formal authorisation of war against the Islamic State militant group.
He played an important role in securing congressional review of the 2015 international deal on Iran's nuclear program, although he eventually backed it.
He has a track record of backing liberal causes such as ending across-the-board automatic budget cuts and providing a pathway to citizenship to millions of undocumented immigrants.
Kaine, a Catholic who became fluent in Spanish while serving as a missionary in Honduras, has expressed personal opposition to abortion, but has a public record in support of abortion rights.