The latest legal challenge to California's voter-approved ban on equal marriage may be a long way from the U.S. Supreme Court, but the first step on a possible path to the high court unfolds this week before a San Francisco federal judge.

Chief U.S. District Judge Vaughn Walker on Thursday will consider arguments on whether to block Proposition 8, which amended the California constitution last November and barred gay and lesbian couples from being legally married in the state.

The high-powered challenge to Prop 8 shifts the legal battleground over gay people in California to the federal courts, in the wake of May's ruling from the California Supreme Court upholding the initiative. The state Supreme Court found it did not have the legal authority to overturn a voter-approved constitutional amendment, although the justices refused to invalidate an estimated 18,000 marriages that were licensed before Prop 8 went into effect.