New York Senator Kirsten Gillibrand will be challenged in the fall by Republican candidate Wendy Long, an ardent opponent of gay marriage.

Long, a former law clerk to U.S. Supreme Court Justice Clarence Thomas, won Tuesday's New York primary, taking 51 percent of the vote.

According to the Associated Press, Long beat her closest rival, Congressman Bob Turner, by 15 percentage points.

Gay marriage foe the National Organization for Marriage (NOM), which previously vowed to overturn New York's marriage equality law, has endorsed Long's Senate aspirations.

Senators Gillibrand and Charles Schumer endorsed Governor Andrew Cuomo's gay marriage bill. Gillibrand also backs repeal of the Defense of Marriage Act (DOMA), which bans gay marriage, and is the sponsor of a bill which would bar discrimination against gay, lesbian, bisexual or transgender people who want to adopt or become foster care parents.

The Human Rights Campaign (HRC), the nation's largest gay rights advocate, endorsed Gillibrand's 2010 re-election.

While voters might not pay much attention, gay marriage foes are nearly certain to bet heavily on Long. Knocking out Gillibrand – a leading advocate of gay rights in the Senate – would help stall repeal of DOMA in the Senate while dealing a psychological blow to gay rights advocates.