Former Democratic presidential candidate Julian Castro's aides are ramping up attacks on Joe Biden and Pete Buttigieg in the wake of Castro's endorsement this week of Sen. Elizabeth Warren of Massachusetts.

While Castro has yet to slam Biden or Buttigieg in his new role as a Warren surrogate, two of his top aides have already taken to social media to land some jabs on the former vice president for saying he’d call for undocumented immigrants to learn English before becoming citizens. They've also hit Buttigieg over race relations in South Bend, Indiana, the city he led as mayor until the beginning of this year.

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“It’s time we dispel the notion of good and bad immigrants based on their ability to speak English,” tweeted Sawyer Hackett, who served as the campaign’s communications director and remains with Castro.

And Castro aide Natalie Montelongo tweeted that “My dad became a naturalized citizen ~25 years ago, and to this day still doesn’t speak English. As a 2nd grader I stayed up countless nights helping him memorize questions/answers in a language he didn’t know. It was hard to see him struggle. It shouldn’t have to be that way.”

The Biden campaign pointed out that the naturalization exam’s verbal portion is in English – except for rare cases – and that their plan offers education support to learn English.

On Wednesday, Hackett took aim at Buttigieg: "A new trove of documents detail widespread racism and discrimination in the South Bend police dept—largely ignored by Mayor Pete. If Pete can’t manage a dept of 300 officers, why should we trust him to run an entire branch of the federal govt?"

It comes as Castro, the former San Antonio, Texas mayor and Housing secretary under then-President Barack Obama, is headed to Las Vegas, Nevada and Marshalltown, Iowa this weekend to stump for Warren.

Castro’s travel as a surrogate to the states that vote first and third in the presidential primary and caucus nominating calendar comes as he transitions into a new role – as a key surrogate for Warren and possibly an attack dog willing to take on two of the senator’s top 2020 nomination rivals.

That’s a role he’s familiar with: Castro clashed with both Biden and Buttigieg a couple of times in the primetime glare of the presidential primary debate stage. And it comes as after engaging in a nasty back and forth with Buttigieg last month on the campaign trail and on the debate stage, Warren appears to be returning to largely avoiding attacking her rivals directly.

A Democratic operative who was close to Castro’s inner circle during his campaign noted that the former candidate’s “bold. He’s not afraid to speak his mind… he will speak his truth.”

But the operative, who asked for anonymity to speak more freely, added that Castro will “do a little bit of contrasts, but I think for him it will be mostly talking about Warren’s record.”

Castro will share the stage with Warren in Iowa on Sunday – which will be their third meeting in just over a week.

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Last Friday -- the day after Castro suspended his campaign -- he quietly met with Warren at her home in Cambridge, Mass., to huddle and to shoot a video with the senator that went viral after it was released on Monday, when the Warren campaign unveiled the endorsement.

The next night, Castro joined Warren at a large rally in New York City.

Castro – who 2016 Democratic nominee Hillary Clinton vetted as her running before eventually choosing Sen. Tim Kaine of Virginia – quickly settled into his new role. He pushed back on the national narrative that Warren – facing slipping campaign cash figures and sliding poll numbers – was faltering.

“So, you know, the polls say this and that. But we still have about four weeks left until the Iowa caucus, and I'm confident that she's going to do very well,” Castro told reporters before he took to the stage.”

And later, he stressed to her supporters in the crowd that “when you talk to people on the ground, you find that they like Elizabeth Warren. You find that they're willing to support her. You find that she has the best damn organization in these early states."

Castro was the only Latino in the large field of Democratic White House hopefuls. And his deployment to Iowa and especially Nevada – where Latino voters play a crucial role in the state’s Democratic presidential caucus – could benefit Warren. It’s also likely he’ll stump for Warren in his home state of Texas – which has a large and vibrant Hispanic electorate. Texas is the second-largest state to vote on the March 3 Super Tuesday contests – the single largest day of voting in the nomination calendar.

But earlier this week the Biden campaign unveiled 10 new endorsements in Texas - including 8 who had backed Castro before he dropped out of the race.