Sen. John Fetterman might not wear a suit, but when it comes to throwing support behind President Joe Biden, he’s more than ready to make a change to his wardrobe. “I’m showing up with brass knuckles,” Fetterman told Huffington Post political reporter Jennifer Bendery Wednesday. “I’m not going to advise the president on anything other than, ‘Stay in. You’re our guy. And I have your back.'” Fetterman was referring to a Thursday meeting between top Biden aides and Democratic Senators concerned about the president’s ability to defeat Donald Trump, the former president and convicted felon, in the …