CINCINNATI - Elizabeth Warren offered an impassioned endorsement of Hillary Clinton on Monday, vouching for her as someone who could be trusted to fight for workers and fend off Donald Trump.

The two most powerful women in the Democratic Party clasped hands and held them high overhead, offering a powerful visual and a preview of what could be a historic presidential ticket.

“Here's what it boils down to. Hillary has brains. She has guts. She has thick skin and steady hands,” said Warren, a champion of the party's liberal base, before 2,600 cheering supporters at the Cincinnati Museum Center at Union Terminal. “But most of all, she has a good heart. And that's what America needs.”

Later, in a speech in Chicago, Clinton turned to the issue that has dogged her for more than a quarter century: Her trustworthiness. She pledged to earn it and defended what some say are too-cautious statements that can sound calculated.

“I personally know I have work to do on this front. A lot of people tell pollsters they don't trust me,” Clinton told the Rainbow PUSH Coalition, adding: “You can't just talk someone into trusting you, you've got to earn it.”

The picture-perfect image in Ohio marked an important moment of party unity after Clinton's long-fought primary against liberal challenger Bernie Sanders, who has not yet endorsed his former rival. And with Warren under consideration to be Clinton's running mate, it may also be a glimpse of the party's future.

Warren showed how she could play attacker-in-chief against Trump, the presumptive Republican nominee, calling him a “small, insecure money-grubber,” ”a nasty man“ and ”goofy.“

An unprecedented two-woman ticket would electrify the party's liberal wing, boosting enthusiasm for Clinton's campaign as she continues to face high unfavourable ratings.

Warren could also help Clinton combat the perception that the multimillionaire former first lady is disconnected from the struggles of working Americans - an image promoted by Sanders during his campaign.

Her arm over Warren's shoulder, Clinton lavished praise on the progressive star, whom she called a “friend” and a “great leader.”

“She is considered so terrific, so formidable, because she tells it like it is,” she said.

Warren returned the compliment, saying of Clinton: “She just remembers who really needs someone on their side and she gets up and keeps right on fighting for the people who need her the most.”

The two women have never been close, according to aides, who note their Senate service didn't overlap and they worked in different corners of the Obama administration. Clinton served as secretary of state, while Warren helped establish the Consumer Financial Protection Bureau.

At times, their relationship has seemed almost frosty. Warren wrote in a 2004 book that as a senator from New York, Clinton “could not afford such a principled position” on legislation that would make it harder for consumers to relieve their debt through bankruptcy laws. She also implied that Clinton was short-tempered and impatient with her staff.

There was little of that distance on Monday. Together, they slammed Trump, casting him as a small-minded, self-interested billionaire who would destroy the country's economy.

“Donald Trump is the guy who wants it all for himself,” said Warren. “And watch out. Because he will crush you into the dirt to get whatever he wants.”

She's taken his hits in return: He blasted her as “Pocahontas,” Republicans claim that Warren fabricated Native American ancestry to help boost her legal career. Warren says she never used her background for unfair advantage.

“Crooked Hillary is wheeling out one of the least productive senators in the U.S. Senate, goofy Elizabeth Warren, who lied on heritage,” Trump tweeted on Monday. Later, he called her “a racist” and “a fraud,” in an interview with NBC News.

Former Massachusetts Sen. Scott Brown, a Republican who lost his Senate seat to Warren in 2012, said Clinton “is considering making someone vice-president who has very serious character flaws when it comes to honesty and credibility” in dealing with her heritage. Republicans say Warren fabricated her ethnic background to gain an employment advantage. Warren denies the accusation. Brown - who moved to New Hampshire to run for Senate in 2014 and lost that race, too - on Monday suggested Warren get a DNA test to clear up the heritage matter.

Warren's tough assault on Trump is valued by Clinton, who aides say particularly appreciates surrogates that don't mince words in their attacks.

“I do just love to see how she gets under Donald Trump's skin,” said Clinton.

Warren has been trying to endear herself to Clinton in other ways. Days after a private meeting at Clinton's home, Warren stopped by her campaign headquarters in Brooklyn to deliver a pep talk to staffers.

She's being vetted by lawyers involved in Clinton's vice-presidential search, and they've asked Warren for documents and to complete a questionnaire. The next step: a private interview with Clinton.

Other candidates under consideration include Virginia Sen. Tim Kaine, a well-liked lawmaker from an important general election battleground state; and Housing and Urban Development Secretary Julian Castro of Texas, a rising star in the Democratic Party.

Clinton remains weeks away from making her pick, according to aides.

Thomas reported from Washington. Associated Press writers Steve Peoples in Washington and Sophia Tareen in Chicago contributed to this report.