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Former San Antonio mayor Julián Castro used some of his spotlight time in Wednesday’s Democratic presidential debate to argue that if Congress doesn't start impeachment proceedings against President Trump, the president will use that to his advantage in 2020.“If they don’t impeach him, he’s going to say, ‘The Democrats didn’t go after me on impeachment, and you know why? I didn’t do anything wrong,’” said Castro, who also served as the Obama Administration's housing secretary. “Conversely, if [Senate Majority Leader] Mitch McConnell lets him off, we’re going to say, 'Sure, they impeached him in the House, but his friend Mitch McConnell — Moscow Mitch — let him off the hook.'"Impeachment proceedings must start in the House before moving to the Senate, where a two-thirds majority is needed to remove a president. That outcome appears unlikely from the Republican-controlled Senate, which has repeatedly shown itself to be in Trump's corner.The impeachment argument was one of several standout moments for Castro, who also surprised pundits with a breakout performance in June during the first Democratic primary debate.On Wednesday, Castro delivered one of the night's most memorable slaps to frontrunner Joe Biden, who also served in the Obama administration. "It looks like one of us has learned the lessons of the past and one of us hasn't," he told the former vice president.Castro also earned applause for calling out New York City Mayor Bill de Blasio on the city's failure to fire the police officer accused of choking Eric Garner to death