WASHINGTON — As gay couples fight for the right to wed, a little-known but determined force is working to stop them: Brian S. Brown, a 39-year-old father of eight who has raised millions of dollars from religious conservatives — especially his fellow Roman Catholics — to become the nation’s leading opponent of same-sex marriage.

Mr. Brown, president of the nonprofit National Organization for Marriage here, was instrumental in passing Proposition 8, California’s same-sex marriage ban, and is working in a host of states (and recently in France) to defeat gay-friendly candidates and outlaw what he calls “the redefinition of marriage.”

Now his views are facing the ultimate American test: a hearing before the Supreme Court, which is considering whether to overturn Proposition 8 and perhaps declare it unconstitutional.

When the justices hear oral arguments in the case on Tuesday, Mr. Brown — unfazed by polls showing that a majority of Americans disagree with him — will be on the court’s marble steps, leading a march that he hopes will draw thousands who believe, as he does, that gay unions hurt children and threaten religious freedom.