The federal amicus brief is unprecedented in matters of marriage access and LGBT rights cases that have come before the Court in recent decades.
For example, no such brief was filed regarding the 1967 landmark case Loving v. Virginia, in which the Supreme Court struck down anti-interracial marriage laws; nor did the federal government take a position on 2003's Lawrence v. Texas, which barred states from criminalizing gay sex.
In fact, the substance of the document bears out Obama's cautious, one-step-behind-the-voters modus operandi by supporting the overturn of Prop 8 while stopping short of calling for an outright ban on all state laws that prevent same-sex couples from marrying.
If anyone could achieve this resounding, nationwide victory, it would be Supreme Court battle veterans Olson and Boies, who argue that any restriction on the right of gays and lesbians to marry violates the equal protection clause of the Fourteenth Amendment.
However, such a sweeping, progressive decision by the Supreme Court in its current makeup is a long shot at best.
Then again, the opposite extreme--a broad ruling upholding Prop 8 and guaranteeing the right of all fifty states to deny marriage rights to same-sex couples--is also unlikely (otherwise, the extremely SCOTUS-savvy Olson and Boies would not have invested years into pursuing this case in the first place).
A third possibility, that the Court will simply dismiss the appeal based on the lack of so-called Article III standing of the Prop 8 proponents, is similarly unlikely. (Why accept such a high-profile case just to throw it back to the District Court?)
That leaves the two most probable outcomes of Hollingsworth: either a narrower ruling overturning Prop 8 on the grounds that, since California already provides every legal state marriage right to same-sex couples except for the title, that semantic exclusion has no purpose other than to perpetuate social stigma against gay people (forcing not only CA, but also seven other states with similar policies, to grant full marriage equality); or the narrowest possible victory, a finding that Prop 8 is unconstitutional because it took away an already existing right to marry (affecting California only).
By advocating for one of these relatively "moderate" decisions instead of pushing for immediate marriage equality for all fifty states, Obama's brief may strike just the right middle-of-the-road chord to sway presumed swing Justice Anthony M. Kennedy against Proposition 8.
Indeed, the federal brief goes so far as to quote a 2001 concurrence authored by Justice Kennedy himself (against the use of "prejudice...[as] the basis for differential treatment under the law") in its argument--surely a calculated maneuver.
President Obama, while hardly an ardent man of principle, is ever the consummate politician.
Perhaps his practice of the Art of the Possible will be just wishy-washy enough to help those at the vanguard of this movement win a significant--if temporarily limited--victory in the ongoing struggle for equal civil rights for all Americans.