Obama’s forceful Inaugural appeal on behalf of the right of gays to marry is being rightly hailed as historic. But make no mistake: There is still another step Obama can take to translate those words into actions that will make it more likely that all gay Americans will enjoy this right under federal law.

Specifically: It still remains to be seen whether the Obama Justice Department will submit a legal brief in the Proposition 8 case being argued before the Supreme Court this spring — one setting forth that the administration believes Prop 8 is unconstitutional. This could have a palpable impact on the outcome — one in keeping with the moral urgency Obama displayed yesterday.

In his remarks yesterday, Obama said this (emphasis mine):

Our journey is not complete until our gay brothers and sisters are treated like anyone else under the law, for if we are truly created equal, then surely the love we commit to one another must be equal, as well.

The key words there are “under the law,” and it’s worth noting that those words may well have been intended for the ears of the Supreme Court justices who sat there looking on as he spoke.

Here’s why. The Supreme Court will weigh in this spring on both the Defense of Marriage Act and the Prop 8 case. The latter is arguably more important. If the Court adopts a sweeping ruling that Prop 8 is unconstitutional because it violates the due process and equal protection clauses, violating the fundamental right to marry, it will open the door to legal challenges to all state laws currently banning gay marriage. Lower courts have issued such broad rulings, and if the Supreme Court agrees, gay rights advocates believe it will only be a matter of time until all state statutes are ruled unconstitutional.

The legal team representing the plaintiffs in the Prop 8 case are lobbying Obama’s Justice Department to file a brief arguing that Prop 8 is unconstitutional, sources say, and the Justice Department is mulling the request. Obama has taken a position against Prop 8 on the policy merits, but his administration has not declared whether it believes the law is constitutional or not. If it does declare its belief that the law is unconstitutional, it could help persuade the Justices that public opinion is inexorably trending towards gay marriage, influencing the case’s outcome.

White House press secretary Jay Carney said today that Obama still believes gay marriage should be an issue for the states. But Obama can believe this while simultaneously claiming he believes Prop 8 crossed a line into unconstitutionality. That would not be an argument for a federal gay marriage statute, but rather only a declaration that this particular state law is unconstitutional.

The argument that the Justice Department should weigh in on Prop 8 is not to say Obama’s words yesterday were unimportant. They were historic, and his record on gay rights has been sterling. It’s only to say that Obama’s own words — that we should not rest until gay couples are treated equally “under the law” — are an eloquent reminder that he should take this next step, and declare whether Prop 8 violates the Constitutional right to equal protection “under the law.”

After all, Obama’s speech yesterday described gay rights as an extension of the battle for civil rights for African Americans — an area where the federal courts did step in and override states-based efforts to deprive blacks of their rights, including in the area of interracial marriage. And Obama’s entire speech yesterday was about translating ideals of freedom into action. Obama’s next action on this front, then, should be very clear.