Hillary Clinton and Sen. Tim Kaine at a rally in Virginia. REUTERS/Carlos Barria Hillary Clinton has selected Sen. Tim Kaine on Friday as her vice-presidential running mate.

The Virginia senator is expected to appear with Clinton in Miami for their first joint appearance on Saturday.

"I'm thrilled to tell you this first. I've chosen Sen. Tim Kaine as my running mate," Clinton told supporters in a text message.

Clinton's selection of Kaine could help bolster the former secretary of state's chances in Virginia, a key swing state. He could also prove an important surrogate to Hispanic voters alienated by Trump's harsh rhetoric about immigration: The senator is a fluent Spanish speaker.

Before serving on the Senate Armed Services Committee as a Virginia senator, Kaine was the state's governor, the mayor of Richmond, and the chair of the Democratic National Committee. He graduated from Harvard Law School in 1983.

Still, liberal critics appear un-enthused by Kaine.

Some have dismissed the senator as boring and not adequately progressive on issues like trade. In a political era where loyalty to ideological orthodoxy is often prized over compromise, Kaine has also worked across the aisles — according to GovTrack, 44% of Kaine's bills and resolutions in 2015 had a Republican cosponsor.

But some criticism of the senator appears more based in his longtime ties to party leadership and low-key personality rather than his actual stances on policies.

Kaine is willing to concede that at least one of those charges may be true.

"They're true. I am boring. But boring is the fastest-growing demographic in this country," Kaine said during an appearance last month on "Meet the Press."

Republicans were quick to condemn Clinton's choice, claiming that Kaine would not appeal to grassroots liberal voters.

"Hillary Clinton's choice of Tim Kaine does nothing to unify a fractured Democrat base which is repelled by her dishonesty and cronyism," RNC Chairman Reince Priebus said.

"After spending last week pandering to grassroots Democrats with Bernie Sanders, Hillary Clinton has chosen someone who holds positions that she's spent the entire primary trying to get to the left of," he said.

Kaine was selected over other top contenders, like Agriculture Secretary Tom Vilsack and Sens. Elizabeth Warren and Corey Booker, for the position.