Story highlights Massachusetts Sen. Elizabeth Warren is on the campaign trail with Democratic presumptive nominee Hillary Clinton

Monday's joint appearance could fuel speculation about Clinton's deliberations over a vice presidential pick

Cincinnati (CNN) Elizabeth Warren unleashed a blistering attack against Donald Trump Monday in her debut on the 2016 campaign trail, delivering a fiery speech that resembled a live audition for the role of Hillary Clinton's top surrogate -- or vice president.

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.

Hillary Clinton and U.S. Sen Elizabeth Warren wave to the crowd before a campaign rally on June 27, 2016 in Cincinnati, Ohio.

"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."

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