PARK CITY, Utah (AP) — Attorneys Ted Olson and David Boies once argued the Bush v. Gore case in front of the U.S. Supreme Court, but they say fighting California's law prohibiting same-sex marriage is the most significant thing they've done.

The two courtroom veterans fought on opposite sides of the case that determined the 2000 presidential election, yet they joined forces to defeat California's 2008 gay-marriage ban, Proposition 8. Their five-year effort is documented in "The Case Against 8," which premiered at the Sundance Film Festival.

The film follows the attorneys and plaintiffs in the lawsuit that resulted in the marriage ban being overturned last year.

Olson said the case was the most important thing he's done "as an attorney or as a person."

The film will air on HBO in June.