Kevin Youkilis, a popular player on Boston's World Series winning teams, was traded to Chicago in a deal for relief pitcher Zach Stewart and utility fielder Brent Lillibridge on Sunday.

“Boston, I just want to say — thank you for Youkilis," Obama told a Boston-area fundraiser Monday night.

"I’m just saying. He’s going to have to change the color of his 'sox,' " the president continued, as the audience laughed and jeered. "I didn’t think I’d get any boos out of here, but I guess I shouldn’t have brought up baseball. I understand. My mistake. You've got to know your crowd."

A member of the audience then called out, "We still love you," drawing more laughter and applause from the assembled crowd.

But the Romney campaign, sensing a potential gaffe, shot out an email to reporters mocking the president. "Last night in Boston, President Obama went to the heart of Red Sox nation and committed an error by taunting fans over the Kevin Youkilis trade to the Chicago White Sox," said spokeswoman Andrea Saul. "The Red Sox have suffered many setbacks over the years — the Babe Ruth trade, the ball through Buckner’s legs, the Bucky Dent home run. Maybe the President should have congratulated the team for winning the World Series in 2004 and 2007. Instead, he chose to mock them for trading away one of its favorite players at a time when the team is struggling."

Earlier Tuesday, White House press secretary Jay Carney defended the president's ribbing of Boston fans.