Rights groups to challenge Arizona immigration law

* Groups: law is unconstitutional, fosters racial profiling

* Furor spurs Democrats' reform efforts in Congress

* Obama warns no 'appetite' for issue before election

By Tim Gaynor

PHOENIX, April 29 (Reuters) - Civil rights groups prepared on Thursday to announce the first legal challenge to Arizona's new immigration law, which has put the issue back on the front burner of U.S. politics.

The American Civil Liberties Union, the Mexican American Legal Defense and Educational Fund and the National Immigration Law Center, which argue the law is unconstitutional, were expected to outline their legal strategy at the Arizona state capitol in Phoenix.

Arizona's Republican governor, Jan Brewer, signed the measure into law Friday, making it a state crime to be in Arizona illegally. It also requires state and local police to determine a person's immigration status if there is "reasonable suspicion" they are undocumented.

In a statement, MALDEF called the law "unconstitutional" and said it "encourages racial profiling, endangers public safety and betrays American values."

Republican backers say the law is needed to curb crime in the desert state, which is home to some 460,000 illegal immigrants and is a key corridor for drug and migrant smugglers from Mexico. This year, the U.S. Border Patrol's Tucson sector has made about 650 arrests a day.

While the measure has sparked an outcry among Latinos, civil rights activists and organized labor, some public opinion polls show broad support for the measure.

BROAD SUPPORT DESPITE FUROR

A Rasmussen Reports poll on Wednesday found that nearly two-thirds -- 64 percent -- of Arizona voters favored the statute. A telephone survey this week showed that 60 percent of voters nationwide backed such a law.

Earlier this week, President Barack Obama's administration said it was considering its own court challenge. The administration has said the law could inflame the immigration debate and divert resources from pursuing those in the country illegally who have committed more serious crimes.

Critics say the law violates the federal government's authority to control immigration.

The furor has revived the immigration debate ahead of November's mid-term congressional elections. And it has ratcheted up pressure on Obama to keep his campaign promise to Hispanics, a key Democratic constituency, to seek passage of comprehensive immigration reform early in his presidency.

Obama said on Wednesday there may not be an "appetite" in Congress to tackle the divisive issue in an election year.

Passing a bill offering a path to citizenship for many of the 10.8 million illegal immigrants in the United States would boost support for Democrats among Hispanics, the country's largest minority, but would run the risk of energizing Republican opposition to Democratic lawmakers in swing states and congressional districts.

The uproar could boost turnout at rallies across the country this weekend. It has spurred calls for economic boycotts of Arizona and its Major League Baseball team. (Additional reporting by David Schwartz; Editing by Steve Gorman and Stacey Joyce)