Can an attorney general do this - urge state attorney generals not to enforce the law?

When it comes to gay marriage, yes.

Reuters:

The United States' top law enforcement official has launched into the divisive gay marriage debate by telling a newspaper his state counterparts do not have to defend laws and bans in court that they think are discriminatory.

U.S. Attorney General Eric Holder's comments to the New York Times came after at least five state lawyers, all of them Democrats, came under fire for refusing to try and defeat legal challenges to bars on same-sex unions in their areas.

Some Republicans and campaigners against gay marriage have criticized the stands taken by the state-level attorneys general, saying they have a duty to defend state law, whether they agree with the policy or not.

But Holder, the nation's first black attorney general who has called gay rights one of "the defining civil rights challenges of our time", drew parallels with legal fights over the racial integration of schools in the 1950s.

"If I were attorney general in Kansas in 1953, I would not have defended a Kansas statute that put in place separate-but-equal facilities," Holder said in an interview published on Monday.

Gay couples and rights supporters have launched a series of legal challenges to bans in federal and state courts in recent months following two key decisions from the U.S. Supreme Court last year.

Attorneys general should base their decision on whether to defend their states in such cases, not on politics, but on questions of guarantees under the U.S. Constitution, such as equal protection of the law, Holder added.

"Engaging in that process and making that determination is something that's appropriate for an attorney general to do," Holder told the New York Times.

A decade ago, only one state was preparing to allow same-sex marriages.

Now around 17 states and the District of Columbia allow same-sex nuptials in a movement that has gained momentum since the nation's top court ruled in June that legally married same-sex couples were eligible for federal benefits.

Attorneys general in states such as Nevada, Oregon, Pennsylvania and Virginia, have said they would not defend gay marriage bans.

Their refusals have not led to the automatic overturning of the bans. In Oregon, for example, the ban stood as it was enshrined in the constitution.

Ordinarily, if a state AG fails to enforce or defend a law in court, he could be brought up on impeachment charges for derliction of duty. I guess gay marriage is a matter beyond the law - or so we're led to believe.

Whether he realizes it or not, Holder is on a very slippery slope with this advice. State AG's that can pick and choose which of their own laws to defend or enforce based on politics would throw the entire concept of the "rule of law" on its head. Why should AG's stop at gay marriage bans? What is to prevent them from interpreting the Constitution in such a way that prevents a state from enforcing any number of laws regarding freedom of religion or speech? I daresay there would be a lot of laws that Holder would want enforced that state AG's would now have the freedom to ignore.

The correct procedure would be to amend or repeal the offending law. Unilateral action by state attorney's general to ignore the legislative process and take matters into their own hands is only asking for trouble.