The news comes from the invaluable SCOTUSblog, which is by far the best source of reporting on the United States Supreme Court.
King joins 12 other state Attorneys General and the attorney general of Washington D.C. in signing onto the brief.
The news of the brief was overshadowed nationally by the Department of Justice amicus brief in support of striking down the same-sex marriage ban.
The brief is on the Hollingsworth v. Perry
Importantly, the states in question include not only the ten jurisdictions that currently allow same-sex couples to marry, but also four states that do not–Delaware, Illinois, New Mexico and Oregon–one of which (New Mexico) does not have a “domestic partnership” or civil union” law providing same-sex couples with all benefits and privileges available to similarly situated opposite-sex couples. Although the brief does not appear to expressly acknowledge that its analysis would call into question the constitutionality of the laws of those four states, that would appear (at least on a quick first read) to be the natural implication.
In a more locally political look, King has already announced that he is running for governor in 2014 and has begun raising money.
Gay marriage is no longer a third-rail of electoral politics as it was in even the recent past. The support of King, the Democratic congressional delegation and many Democratic members of the state legislature for gay marriage shows that attitudes have changed on this issue.
The Democratic Party of New Mexico also endorses marriage equality in its official party platform.
For the most part, Republicans have remained opposed to same-sex marriage on both a state and federal level.
One notable exception is former New Mexico Gov. Gary Johnson who himself signed onto an amicus brief in favor of same-sex marriage earlier this week..
For readers more well-versed in the law, here is the brief itself.
Headline updated for accuracy.