Arizona Gov. Jan Brewer signed into law Friday the most restrictive immigration bill in the country, setting the stage for a showdown with the Obama administration and reigniting a divisive national debate less than seven months before congressional midterm elections.

Brewer, a Republican facing a stiff primary challenge, said she had no choice but to act because Washington’s failure to address the issue had effectively left border protection to the states. “We in Arizona have been more than patient waiting for Washington to act,” she said, as hundreds of demonstrators gathered outside her Phoenix office. “But decades of federal inaction and misguided policy have created an unacceptable situation.”

Even before it was signed, President Obama criticized the Arizona law, which requires police to question anyone who appears to be in the country illegally. Obama called the effort “misguided” and directed the Justice Department to monitor its implementation, warning that it could violate citizens’ civil rights. Immediate legal challenges were expected from outside groups.

Obama cited the measure as a sign that Congress must act swiftly on overhauling immigration, saying failure to do so would “only open the door to irresponsibility by others.”

With the stroke of a pen, Brewer unleashed the passions of activists and politicians on both sides of the issue. Hispanics across the country, a key political bloc, promised an energetic push to elect Democrats in November. House Speaker Nancy Pelosi (D-Calif.), an advocate of immigration reform, issued a statement describing the law as “harsh.”

The response among national Republican lawmakers was more muted, reflecting a split over whether to pursue stricter immigration laws or to court the expanding pool of immigrant voters.

Under Arizona’s new law, to take effect in 90 days, it will be a state crime to be in the country illegally, and legal immigrants will be required to carry paperwork proving their status. Arizona police will generally be required to question anyone they “reasonably suspect” of being undocumented — a provision that critics argue will lead to widespread racial profiling, but that supporters insist will give authorities the flexibility to enforce existing immigration laws.

Obama’s opposition

On Friday, Obama voiced opposition to the bill for the first time at a naturalization ceremony for two dozen foreign-born members of the U.S. military. He urged the country to “choose a different future” than the one envisioned in the Arizona legislation. Although he said the Justice Department would “closely monitor” developments, Obama stopped short of demanding immediate intervention.

Joining Obama at the Rose Garden event was Homeland Security Secretary Janet Napolitano, who vetoed similar bills repeatedly during two terms as Arizona’s Democratic governor. She said she did so because “they would have diverted critical law enforcement resources from the most serious threats to public safety.”

Recent events have underscored Arizona’s role as a roiling cauldron of immigration politics: Brewer has been under pressure to sign the bill from state Treasurer Dean Martin, who is considered her biggest threat in the Aug. 24 Republican primary. Sen. John McCain (R), in his own tough primary for reelection, only recently came out in support of the bill — and, on Friday, did not issue a statement. His opponent, former congressman J.D. Hayworth, praised Brewer in a statement and attacked McCain “and others serving in Washington [for having] alternated between inaction and amnesty.”

The measure goes far beyond a controversial federal program that provides grants and training to about 70 state and local police agencies to enforce immigration laws. Frederick County, Md., and several jurisdictions in Virginia, including Prince William County, have joined that 287(g) program, which is named for a section of federal law. Under another program, state and local jurisdictions in the Washington area and nationwide check fingerprints of people booked into local jails against federal immigration databases.

Senate Majority Leader Harry M. Reid (D-Nev.), having concluded that talks to advance a bipartisan immigration bill were stalled, recently told Sens. Charles E. Schumer (D-N.Y.) and Lindsey O. Graham (R-S.C.) that if they cannot strike a deal within three weeks, Democrats will bring their own bill forward, aides and lobbyists said.

Many lawmakers on both sides, familiar with how treacherous immigration reform proved when President George W. Bush sought it and failed, say they are uncertain about its prospects.

Some Democrats have calculated that even if an immigration bill fails, a debate on it could rally their base and mobilize Hispanic voters against GOP lawmakers in some districts. And while it could also energize Republican voters, some Democrats said the Arizona bill has also provided them with the opportunity to put Republicans on the defensive nationally.

Earlier attempts

The legislation makes Arizona the first state to criminalize illegal immigration by defining unlawful presence as trespassing, according to the National Council of State Legislatures. Between 2007 and 2009, California, Colorado and Texas considered similar proposals but killed them.

However, frustration in the states continues to drive legislatures to pass a patchwork of laws each year, some cracking down on illegal immigrants and others seeking to prevent exploitation of them by human traffickers and unscrupulous employers. The NCSL estimates that about 1,400 bills are introduced each year.

“There’s a huge concern, and states have been down this path before, where they have been taken to court and that has been very costly. . . . This will be very much a wait-and-see approach,” said Ann Morse, director of the NSCL immigration policy project.

The law could also have diplomatic and economic repercussions. The Mexican Senate voted unanimously to urge Brewer to veto the bill, saying it could lead to persecution and harassment of Hispanics, and the country’s Ministry of Foreign Affairs expressed great concern and said the measure could harm cross-border relations “for generations.”

At the White House and the Justice Department, lawyers on Friday began examining the Arizona law to see if questions about racial profiling might require federal intervention. The Justice Department’s Civil Rights Division and its Civil Division, which represents the United States in civil litigation matters, are expected to conduct a joint review.