Sen. Elizabeth WarrenElizabeth Ann WarrenOvernight Finance: Lawmakers grill Equifax chief over hack | Wells Fargo CEO defends bank's progress | Trump jokes Puerto Rico threw budget 'out of whack' | Mortgage tax fight tests industry clout Michelle Obama is exactly who the Democrats need to win big in 2020 Wells Fargo chief defends bank's progress in tense Senate hearing MORE (D-Mass.) went on the attack for Hillary ClintonHillary Diane Rodham ClintonChris Murphy’s profile rises with gun tragedies DNC, RNC step up cyber protections Gun proposal picks up GOP support MORE on Thursday night, ripping into Donald TrumpDonald John TrumpIvanka Trump pens op-ed on kindergartners learning tech Bharara, Yates tamp down expectations Mueller will bring criminal charges Overnight Cybersecurity: Equifax security employee left after breach | Lawmakers float bill to reform warrantless surveillance | Intel leaders keeping collusion probe open MORE, Clinton's rival for president, as a "thin-skinned, racist bully."

Shortly before announcing her endorsement of Clinton on MSNBC, Warren laid into Trump during a speech at the American Constitution Society convention in Washington.

She slammed Trump for claiming that an Indiana-born federal judge was biased because of his Mexican heritage and his suggestion that a Muslim judge would be similarly unable to be fair based on Trump's proposals. 
“He has even condemned federal judges who are Muslim on the disgusting theory that Trump’s own bigotry compromises the judges' neutrality," she said. "You just can't make this stuff up."
“What kind of a man is Donald Trump? Donald Trump says Judge Curiel should be ashamed of himself. No, Donald, you should be ashamed of yourself," Warren said. "You shame yourself and you shame this great country."
She pointed out that Judge Gonzalo Curiel, who is overseeing a fraud lawsuit against Trump University, is unable to defend himself against Trump's attacks.
"Trump is picking on someone who is ethically bound not to defend himself — exactly what you would expect from a thin-skinned, racist bully," Warren said.
Warren also attacked Trump's businesses, which he often points to as evidence of his leadership capabilities, and said ventures like Trump University show he doesn't think the rules apply to him. 
"Donald Trump is a loud, nasty, thin-skinned fraud who has never risked anything for anyone and who serves no one but himself," she said.
"Trump tells everyone who will listen that he is a great businessman. But let's be honest, he's just a guy who inherited a fortune and kept it rolling along by cheating people," she said. "And you know, when that's your business model, sooner or later you're going to run into legal trouble, and Donald Trump has run into a lot of legal trouble."
Earlier this week, Ryan (R-Wis.) called Trump's comments about Curiel "the textbook definition of a racist comment" but has indicated he'll support Trump over Clinton.
Warren also singled out Sen. Jeff SessionsJefferson (Jeff) Beauregard SessionsIntel leaders: Collusion still open part of investigation Republicans jockey for position on immigration Biden to Alabama: No more extremist senators MORE (R-Ala.), Trump's biggest ally in the Senate, saying he's "attacked the integrity of several of President Obama's nominees" because of their ties to the American Civil Liberties Union. 
"From the day President Obama was sworn in, Senate Republicans have used every procedural tool at their disposal to try slow down his nominees," she said. 
Calling the tactics "outrageous," Warren repeatedly said to applause from the audience: "Senate Republicans, do your job." 
The comments come after Warren tore into McConnell (R-Ky.) for blocking judicial nominees, saying he "forces his will" on the Senate. 
Speculation has mounted over whether Clinton will pick Warren as her running mate ahead of a battle with Trump. Clinton said Thursday that Warren would be "eminently qualified" for any position.
Trump took to calling Warren "Pocahontas" on the campaign trail starting weeks ago, going after Warren's old claim of Native American ancestry, and has said he wants Clinton to pick her as vice president, predicting a victory over the two.
Jordain Carney contributed