Former Housing and Urban Development Secretary Julián Castro jabbed at former Vice President Joe Biden Joe BidenPelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Hillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns Fox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio MORE for his opposition to decriminalizing illegal border crossings in a tense exchange at the Democratic presidential debate in Detroit on Wednesday night.

Castro, who worked with Biden in the Obama administration, has proposed decriminalizing border crossings by reducing it to a civil offense.

Critics of President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE say he has used an obscure law that makes illegal border crossings a federal offense as justification for his "zero tolerance" border crossing policy that has resulted in family separations.

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On Wednesday night, Biden said that Castro never expressed a disagreement with Obama-era policies on immigration until he started running for president.

“One of us has learned the lessons of the past and one hasn’t,” Castro shot back. “The only way we’re going to guarantee these family separations don’t happen in the future is to repeal the law.”

The crowd cheered the exchange.

Obama’s former Homeland Security secretary, Jeh Johnson, has been making the rounds arguing that Castro’s plan will attract scores of new immigrants to the border and will be an election loser for Democrats.

Biden took up that argument on Wednesday night.

“I have guts enough to say his plan doesn’t work,” Biden said.