Jerry Markon and Anne Kornblut offer "Justice Department considers suing Arizona to block immigration law" (link):

Officials in the Obama administration are urging the extraordinary step of suing Arizona over its new immigration law, and the Justice Department is considering such an action to block the legislation from taking effect, government officials said Wednesday.

A key legal ground being considered, officials said, is the doctrine of "preemption" -- arguing that the state's law illegally intrudes on immigration enforcement, which is a federal responsibility...

The prospect of federal lawyers marching into court to challenge a state law would be most unusual, legal specialists said. Typically, the government files briefs or seeks to intervene in lawsuits filed by others against state statutes; federal officials said that could still happen in the Arizona case...

...Kris Kobach, who helped draft the legislation, said similar preemption arguments failed when Arizona passed a 2007 law that sanctioned employers for knowingly hiring illegal immigrants. "They tried this on for size already, and it failed," said Kobach, a senior Justice Department official in the George W. Bush administration who is now a constitutional law professor at the University of Missouri at Kansas City.

Needless to say, this would set off a political firestorm of a sort, pitting the illegal immigration-supporting establishment against the will of around 70% of a state. The great majority of conservatives and Republicans would be enraged as would a good portion of Democrats. The only ones on the Obama administration's side would be the usual suspects: those who seek to profit from illegal activity in one way or another. It's more likely that, instead of a full frontal assault, they'd help behind the scenes, such as providing assistance to the Mexican government-linked American Civil Liberties Union or the like.

Note also that Kornblut and Markon either lied or got wrong part of the Arizona law:

The Arizona law criminalizes illegal immigration by defining it as trespassing... The measure makes Arizona the first state to criminalize illegal immigration by defining it as trespassing, according to the National Conference of State Legislatures.

In fact, that trespassing provision was struck from the bill as signed into law, and even the New York Times has already corrected a similarly false Linda Greenhouse OpEd.