The Human Right Campaign (HRC), the nation's largest gay rights advocate, has pulled its endorsement of Illinois Rep. Mark Kirk after he voted against repeal of “Don't Ask, Don't Tell,” the 1993 law that forbids gay troops from revealing their sexuality.

Instead, the group threw its support behind Illinois Treasurer Alexi Giannoulias, Kirk's Democratic challenger for the Illinois Senate seat being vacated by Democrat Roland Burris.

Kirk, considered a moderate gay ally for his backing of a federal hate crimes law that includes sexual orientation and gender identity and the Employment Non-Discrimination Act (ENDA), a bill that would outlaw workplace LGBT discrimination, won the group's backing over an openly gay Democratic candidate in 2004.

On Thursday, the Naval Reserve officer voted against attaching to the defense budget an amendment that would repeal the gay ban.

“While many lawmakers stood on the right side of history in voting to end Don't Ask, Don't Tell,' Rep. Kirk voted to maintain discrimination that hurts our national security,” Joe Solmonese, president of HRC, said in announcing the group's endorsement.

The following day, Kirk voted in favor of the $700 billion defense bill, which included the repeal amendment, one of the few House Republicans to do so.

“Alexi Giannoulias is the progressive leader Illinois needs,” Solmonese said.

“As the next senator from the great state of Illinois, I will lead the fight for equality – for marriage equality, for an end to DOMA, for employment non-discrimination, and for immigration reform that treats same-sex couples fairly,” Giannoulias said.

The endorsement could prove critical as candidates attempt to pull ahead in the neck-in-neck race.

Ironically, Kirk was accused of being gay and in the pocket of gay rights groups by Andy Martin, his GOP primary opponent, in January. Allegations Kirk has denied.