Experts have attributed the decrease to several factors, including the economic downturn, tighter border security and state immigration laws. A 2007 Arizona law, allowed to take effect last year by the Supreme Court, prohibits employers from knowingly hiring illegal immigrants.
But in Arizona and elsewhere, the appetite for new immigration measures appears to have waned, in part because business leaders have objected. Arizona voters ousted Republican state Sen. Russell Pearce, the architect of the 2010 law and the driving force behind other Arizona immigration laws, in a November recall election.
"There has been a great deal of buyer's remorse in those states that have enacted Arizona-type legislation," the ACLU's Romero said.
The high court decision will land in the middle of a presidential campaign in which Obama has been heavily courting Latino voters and presumptive Republican nominee Mitt Romney has been struggling to win Latino support after a drawn-out primary campaign in which he and the other GOP candidates mostly embraced a hard line to avoid accusations that they support any kind of "amnesty" for illegal immigrants living in the U.S.
Justice Elena Kagan sat out last year's case and also will not take part in the new immigration case, presumably because of her work in the Obama administration. The court's conservative majority held sway in last year's 5-3 decision.
Associated Press writer Jacques Billeaud in Phoenix contributed to this story.