Slamming Trump as a "a small, insecure money grubber" who would "crush you into the dirt," Warren sought to highlight sharp contrasts between the two parties' presumptive presidential nominees.
"Now, Donald Trump says he'll make America great again ... It's stamped on the front of his goofy hats," Warren said. "You want to see goofy? Look at him in that hat."
Warren's speech offered a clear preview of the high-profile role the senator is expected to play in the general election as Trump's chief public antagonist. Striking the same populist tone that's core to her political fame, Warren said Trump is only in it for himself -- and willing to hurt middle class Americans for his own personal gain.
"When Donald Trump says 'great,' I ask: 'great for who, exactly?'" she said. "When Donald says he'll make America great, he means greater for rich guys just like Donald Trump. That's who Donald Trump is. ... And you have to watch out for him, because he'll crush you into the dirt."
Warren warned: "You have to watch out for him, because he'll crush you into the dirt to get whatever he wants."
Warren and Clinton's joint appearance on Monday was visually striking. The two women, who are only one year apart in age, stepped onto the stage wearing suits of similar shades giving the event a coordinated feel: Warren's outfit a bright royal blue and Clinton's a deep purple.
The image of the duo standing next to one another will further fuel speculation about Clinton's deliberations over a vice presidential pick. Sources say Warren is among the handful of Democrats that the Clinton campaign is scrutinizing as it prepares to choose a running mate
ahead of the Democratic convention next month.
Clinton, for her part, did little to tamp down the rumors. She offered Warren effusive praise and openly acknowledged her surrogate's unique capacity to take on Trump.
"You just saw why she is so terrific, so formidable. Because she tells it like it is," Clinton said. "I have to say, I do just love how she gets under Donald Trump's skin."
The two Democrats have not always shared a warm relationship. But on Monday, Clinton suggested on stage that they shared some personal bonds.
"Elizabeth and I came of age around the same time," Clinton said. "And when we were growing up, as you heard her talking about her parents, her brothers, we believed in the American dream."
The former secretary of state continued to drive home the narrative that Trump is temperamentally unfit to be president.
"Imagine Donald Trump sitting in the Oval Office the next time America faces a crisis," Clinton said. "Imagine him being in charge when your jobs and savings are at stake. Imagine him trying to figure out what to do in case of an emergency."
Trump preempted the event with a dig at both Warren and Clinton.
"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
. He avoided his previously used nickname for Warren, "Pocahontas," when referencing a 2012 controversy over how she characterized her Native-American heritage.
In the afternoon, Trump released a statement calling Warren a "sellout," pointing to the campaign contributions Clinton has received from Wall Street.
"While Warren claims that Wall Street businesses have too much influence in D.C.," the statement read, "the Clinton campaign has accepted over $41 million this cycle from Wall Street interests."
Warren's decision make a formal endorsement and be an active surrogate is a significant political win for Clinton. Clinton's hard-fought battle against Vermont Sen. Bernie Sanders in the Democratic primary spotlighted her struggle to win over liberal factions of her party and Warren's formal backing
earlier this month signaled an important stamp of approval.
Elected to the Senate in 2012, Warren is one of Congress' most outspoken critics of Wall Street and advocate of stringent financial regulations. The former law professor rose quickly to national stardom and holds powerful sway among progressive activists.
Warren's massive liberal fan base crafted a national campaign to recruit the populist senator to run for the White House -- a request that she repeatedly declined.
Now, the liberal firebrand is poised to seize the national platform and emerge one of Trump's most potent adversaries.
"I am here today because I'm with her," Warren said on Monday. "Hillary Clinton will be the next president because she knows what it takes to beat a thin-skinned bully who is driven by greed and hate."