Illinois Republican Party Chairman Patrick Brady has resigned in the face of sustained opposition from conservatives over his support for gay marriage.

"I've been going at it hard for six years. I need to focus on my family, and obviously I had lost the support of the state Central Committee because of my position on gay marriage," Brady told CNN. He led the party for close to four years, during which a Republican was elected to the Senate seat formerly held by President Obama. That senator, Mark Kirk, recently endorsed gay marriage himself.

Earlier this year, Brady endorsed legislation legalizing gay marriage and urged state lawmakers, publicly and privately, to do the same. After the bill passed the state Senate with only one GOP vote, he declared that his party was "on the wrong side of history."

State Sen. Jim Oberweis and other conservative members of the state central committee responded by attempting to oust Brady as party chair. They failed twice, but the state central committee last month approved a succession plan for Brady and began searching for a replacement.

In a resignation letter Brady touted GOP successes during his tenure and thanked Kirk, the state congressional delegation and the Republican leaders in the state House and Senate. He also thanked his wife, Julie, who is battling cancer.

Vice Chairwoman Carol Donovan is serving as acting chair of the state party.

Gay marriage is awaiting a vote in the Illinois state House, where two Republicans have endorsed same-sex unions.