Julián Castro is fundraising off of the criticism he's facing after mounting an all-out attack on former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE during Thursday’s Democratic presidential debate, casting it as “the biggest challenge” his candidacy has ever come up against.

“I had a critical choice to make on the debate stage last night,” Castro, a former Housing and Urban Development (HUD) secretary, said in an email to supporters. “I could either play it safe and give Vice President Biden a free pass like everyone else. Or I could speak up, challenge the conversation, and demand answers for you and your family.”

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"I’m not in this race to play it safe. This fight — our fight — is too important to me to stay quiet when what I hear doesn’t add up,” he continued. "I spent every second on that stage fighting for you — and now I’m being attacked for it.”

“This is now the biggest obstacle I have ever faced. Can I count on you to take a stand for me with $5?”

The email shows how Castro is trying to capitalize on a controversial exchange with Biden that thrust the 76-year-old former vice president’s age into the spotlight once again.

During a debate over health care on Thursday night, Castro insisted that Biden had contradicted himself minutes earlier on the issue of whether some people would have to buy into a health care plan under his proposed system.

“Are you forgetting what you said just two minutes ago?” Castro asked Biden on stage. “I can’t believe that you said just two minutes ago that you had to buy in and now you’re forgetting that.”

In fact, Biden did not appear to have said that people who have to buy into his plan, but rather that if someone lost their job and employer-sponsored insurance, they could potentially opt into Medicare.

Castro’s remarks were widely seen as an attack on Biden’s age. If elected, Biden would be the oldest person to ever sit in the Oval Office, and opponents of the former vice president have repeatedly raised his age as a potential concern.

Castro denied that his comments were meant as “a shot at his age.” Still, the former HUD secretary came under criticism for the attack, with pundits and even some of his fellow presidential hopefuls denouncing the remarks as petty and inappropriate.

"It feels like something that Donald Trump might tweet," Sen. Amy Klobuchar Amy Klobuchar3 reasons why Biden is misreading the politics of court packing Social media platforms put muscle into National Voter Registration Day Battle lines drawn on precedent in Supreme Court fight MORE (D-Minn.) told CNN after the debate, adding that the comments were “so personal and unnecessary.”

For Castro, the attack on Biden appeared to be something of a gamble intended to boost his standing in the primary contest. While he qualified for the debate stage, he has been polling in the low single-digits, well below the race’s top-tier candidates, like Biden, Sen. Bernie Sanders Bernie SandersSirota reacts to report of harassment, doxing by Harris supporters Republicans not immune to the malady that hobbled Democrats The Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election MORE (I-Vt.) and Sen. Elizabeth Warren Elizabeth WarrenOvernight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds On The Money: Half of states deplete funds for Trump's 0 unemployment expansion | EU appealing ruling in Apple tax case | House Democrats include more aid for airlines in coronavirus package Warren, Khanna request IG investigation into Pentagon's use of coronavirus funds MORE (D-Mass.).

There’s evidence that such a strategy can work. Sen. Kamala Harris Kamala HarrisTexas Democratic official urges Biden to visit state: 'I thought he had his own plane' The Hill's Campaign Report: Biden on Trump: 'He'll leave' l GOP laywers brush off Trump's election remarks l Obama's endorsements A game theorist's advice to President Trump on filling the Supreme Court seat MORE (D-Calif.) surged in the race after a heated exchange with Biden in the first Democratic debate in June, when she hammered the former vice president over his opposition to federally mandated school busing as a senator in the 1970s.