Senator Kirk's 'bro with no ho' remark caught on live microphone

CHICAGO (WPVI) -- Controversial comments by Senator Mark Kirk were caught on an open microphone.

He claims he was just joking with a colleague when he used a word many consider derogatory toward women.

On Thursday, there are calls for an apology.

Not only is U.S. Senator Mark Kirk facing a tough re-election battle next year, his Democratic opponent is likely to be a woman. That's more the reason what he said could have political consequences.

The comment was part of the Washington buzz surrounding the Republican presidential candidacy of Senator Lindsey Graham, of South Carolina. Graham is a bachelor, who told a newspaper that if elected, he'd have a "rotating" First Lady.

Graham's colleague, Illinois' Mark Kirk, also a bachelor, whispered into an open microphone during an appropriations committee vote, and said: "I've been joking with Lindsey. Did you see that? He's going to have a rotating First Lady. He's a bro with no ho."

First-termer Kirk, who suffered a stroke in 2012, apparently did not realize his voice was being recorded. His office issued a statement:

"Senator Kirk was joking with his colleague and immediately apologized to anyone offended by his remark."

The 55-year-old Republican is campaigning for re-election in 2016. He has no primary challengers.

But two women, U.S. Representative Tammy Duckworth and Attorney Andrea Zopp, are vying in the Democratic primary.

"Senator Kirk's joke is as offensive as it is unfunny, and he should apologize, personally and immediately," Duckworth's campaign spokeswoman wrote.

"Sen. Kirk, referring to women as a 'ho' is never cute + beneath dignity of a Senator. US women deserve an apology. #StayClassy," Congressman Luis Gutierrez, of Chicago, tweeted Thursday afternoon.

A spokesperson for the Duckworth campaign piled on, alleging that Senator Kirk has voted against equal pay for women and affordable child care.

The senator was unavailable for comment.