Hillary Clinton and Sen. Tim Kaine, Democrat of Virginia, in Annandale, Va., on July 14. (Photo: Saul Loeb/AFP/Getty Images)

ANNANDALE, Va. — Potential vice presidential pick Sen. Tim Kaine called Donald Trump a “me first” trash-talking candidate at a rally in Northern Virginia with Hillary Clinton Thursday.

Kaine is considered a frontrunner on Clinton’s list of potential vice president picks, but progressives in the party have raised concerns he is a “safe” choice who lacks the liberal cred and ability to motivate the base of Sen. Elizabeth Warren and others on the short list.

He debuted his attack lines on Trump Thursday, contrasting Trump to Clinton. “Do you want a trash-talker president or a bridge-builder president?” he asked the crowd. “He trash-talks women, he trash-talks folks with disabilities, he trash-talks Latinos.” He then asked if the crowd wanted a “you’re fired” president or a “you’re hired” president, a “me first or kids and family first” president.

Kaine’s attack lacked the ferocity of Warren’s, who called Trump “a small, insecure money-grubber” who will “crush you into the dirt to get whatever he wants” when she appeared with Clinton in May. (At one point, Kaine said it “gets me steamed” when Trump called the U.S. military a disaster, sounding a bit like a character from a 1950s sitcom.) But Clinton doesn’t necessarily need her vice president to be an attack dog, given she has Warren, Sen. Bernie Sanders and both President Obama and Vice President Joe Biden in her corner and eager to go after Trump. Kaine radiates niceness, which could help the candidate with her trust issues with voters, and he’s fluent in Spanish, a boon with Latino voters. (“I’m boring,” he admitted in a recent interview, before joking that boring people are the fastest growing demographic in America.)

The senator and former Virginia governor, who spent a year as a missionary in Honduras, showed off his Spanish at the beginning of the rally, saying “Estamos listos para Hillary” — we are ready for Hillary. He then explained the subtle difference between the meaning of the word “ready” in English and Spanish.

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Clinton referenced Kaine several times in her speech, saying he was “absolutely right” to frame Trump as a president who is out for himself. “’Do you want a trash-talker or a bridge builder?’ I like that one a lot,” she said.

Dawn Kirton, a travel agent and Clinton supporter who attended the rally, said she thought Kaine was “mediocre” but that Clinton was a “superwoman.” She said she wanted Clinton to pick Warren as her vice president. “She’s a shaker and a mover,” Kirton said.

“I thought he was pretty good,” said Janet Schreiber, an independent voter who supports Clinton. “Very pleasant.”

Hillary Clinton and Sen. Tim Kaine (Photo: Saul Loeb/AFP/Getty Images)

Some progressives in the party say they don’t like Kaine’s past support for the Trans-Pacific Partnership (TPP) trade deal and his personal opposition to abortion. They would like Clinton to pick a more liberal running mate to further rally Sanders’ supporters to her side for the general election.

“So far the Clinton campaign has been surprisingly bold in the progressive positions they’ve carved out,” said Adam Green, founder of the Progressive Change Campaign Committee. “And to go an extremely cautious route with a VP pick would cut against their pattern so far.”

Rep. Keith Ellison, D-Minn., who co-chairs the Congressional Progressive Caucus, told Reuters he would prefer a running mate ” who’s really going to signal to the progressive base that we’re going to make some advances on this income inequality people have been suffering from.”

Choosing Kaine right before the Democratic National Convention, which begins in Philadelphia on July 25, could also put a damper on the party’s unity attempts.

But others downplayed the importance of a progressive vice presidential pick for Clinton.

“The most important moment until late fall is really Clinton’s nomination acceptance speech, whether the VP is announced a week before that or three weeks before that, all that becomes ancient history once the nominee takes the stage and lays out the argument,” said Ben Winkler, Washington director at the grassroots activism group MoveOn.

Michael Feldman, who was a top aide to Al Gore, said there will be plenty of time for Clinton’s team to introduce Kaine to voters if he is chosen. “At the end of the day, it’s the top of the ticket that matters,” he said. “The choice is going to be Clinton somebody versus Trump somebody. And there’s going to be a lot at stake there.”