By Bill Mears, CNN Supreme Court Producer

Washington (CNN) - In a bold political and legal move, the Obama administration formally expressed its support for same-sex marriage in California, setting up a high stakes political and constitutional showdown at the U.S. Supreme Court over a fast-evolving and contentious issue.

In a broadly worded legal brief on Thursday that senior government sources said had President Barack Obama's personal input and blessing, the Justice Department asserted gay and lesbian couples in the nation's most populous state have the same "equal protection" right to wed and that voters there were not empowered to ban it.

"Use of a voter initiative to promote democratic self-governance cannot save a law like Proposition 8 that would otherwise violate equal protection," said the brief. "Prejudice may not however be the basis for differential treatment under the law."

California's 2008 Proposition 8 referendum revoked the right of same-sex couples to wed after lawmakers and the state courts previously allowed it.