By including gay rights in the arc of the struggle for civil rights — the road “through Seneca Falls and Selma and Stonewall” — President Obama linked his presidency to ending antigay discrimination and underscored the legal wrong of denying gay people the freedom to marry.

“Our journey is not complete until our gay brothers and sisters are treated like anyone else under the law,” Mr. Obama famously said in his second Inaugural Address, “for if we are truly created equal, then surely the love we commit to one another must be equal as well.”

Now that Mr. Obama has declared that he believes denying gay people the right to wed is not only unfair and morally wrong but also legally unsupportable, the urgent question is how he will translate his words into action. To start, he should have his solicitor general file a brief in the Proposition 8 case being argued before the Supreme Court in March, saying that California’s voter-approved ban on same-sex marriage is unconstitutional.

Just a day after the inauguration, Mr. Obama’s spokesman, Jay Carney, said that while Mr. Obama supports same-sex marriage as a policy matter, the president still believes it is an issue for individual states to decide. That was Mr. Obama’s formulation when he first announced his support for same-sex marriage in May, and even then it made no sense, except perhaps as political cover approaching the general election campaign.