WASHINGTON - The Obama administration on Thursday declared that gay marriage can be a right that deserves constitutional protection, supercharging a Supreme Court battle that started with California voters and is now shooting for the history books.

Shedding its earlier caution, the administration forcefully asserted in a key legal brief that the Constitution's equal protection guarantees extend to same-sex couples seeking a California marriage license. The administration wasn't required to take a position.

"Proposition 8, by depriving same-sex couples of the right to marry, denies them the dignity, respect and stature accorded similarly situated opposite-sex couples under state law," Solicitor General Donald Verrilli Jr. wrote.

The 33-page brief signed by Verrilli thrusts the administration into the potentially landmark gay marriage case to be heard by the court on March 26.

It potentially puts the administration on the opposite side of 37 states that expressly prohibit same-sex marriage through either a statute or a provision in the state's constitution.

Subtly, though, the brief indicates that the Supreme Court can focus on the "particular circumstances" found in California and seven other states that, likewise, grant domestic partnership rights, but not full marriage benefits. The designation of marriage, Verrilli noted, "confers a special validation of the relationship between two individuals and conveys a message to society that domestic partnerships or civil unions cannot match."

More broadly, the administration's argument is that any laws based on sexual orientation require "heightened scrutiny" from courts.

California in 2008 joined the roster of states that banned gay marriage when voters, by a 52 percent to 48 percent margin, approved the ballot measure amending the state's constitution to say that "only a marriage between a man and a woman is valid or recognized." The proposition trumped an earlier state Supreme Court ruling that concluded same-sex couples had a constitutional right to marry.

"Proposition 8's withholding of the designation of marriage is not based on an interest in promoting responsible procreation and child-rearing, (the) central claimed justification for the initiative, but instead on impermissible prejudice," Verrilli wrote.

Verrilli cited a "significant history of discrimination" against gays and lesbians, as well as the observation that gays and lesbians are a "minority group with limited power to protect themselves from adverse outcomes in the political process."

Family Research Council President Tony Perkins denounced the administration's move as a "brazen" flip-flop caused by pressure from "supporters of marriage redefinition."