WASHINGTON - Under pressure to take action, President Barack Obama on Tuesday ordered 1,200 National Guard troops to boost security along the U.S.-Mexico border, pre-empting Republican efforts to force a congressional vote to send the troops.

Obama will also request $500 million for border protection and law enforcement activities, according to lawmakers and administration officials.

The president's action comes as chances for comprehensive immigration reform, Obama's long-stated goal, look increasingly dim in this election year. Obama has been all but compelled to do something since Arizona's passage of a tough illegal-immigration law thrust the border problem into the public spotlight.

Indeed, Arizona Gov. Jan Brewer credited her signing of the controversial new law for compelling Obama to act. Signing the law, Brewer said in a statement, "clearly ignited the talk of action in Washington for the people of Arizona and other border states."

The National Guard troops will work on intelligence, surveillance and reconnaissance support, analysis and training, and support efforts to block drug trafficking. They will temporarily supplement Border Patrol agents until Customs and Border Protection can recruit and train additional officers and agents to serve on the border, according to a letter Tuesday from top administration security officials to Senate Armed Services Committee Chairman Carl Levin, D-Mich.

In 2006, President George W. Bush sent thousands of troops to the border to perform support duties that tie up immigration agents. But that program has since ended, and politicians in border states have called for troops to be sent to curb human and drug smuggling and to deal with Mexico's drug violence that has been spilling over into the United States.

The White House released the letter signed by national security adviser James Jones and White House counterterror chief John Brennan not long after Obama met at the Capitol with Republican senators who pressed him on immigration issues, including the question of sending troops to the border.

Arizona Sens. John McCain and Jon Kyl have been urging such a move, and Republicans planned to try to require it as an amendment to a pending war spending bill.

In a speech Tuesday on the Senate floor, McCain said the situation on the U.S.-Mexico border has "greatly deteriorated." He called for 6,000 National Guard troops to be sent, and he asked for $250 million more to pay for them.

"I appreciate the additional 1,200 being sent ... as well as an additional $500 million, but it's simply not enough," McCain said.

McCain's amendment would direct the defense secretary to deploy the National Guard troops. The letter from Jones and Brennan took scathing aim at that approach.

"It represents an unwarranted interference with the commander in chief's responsibilities to direct the employment of our armed forces," Jones and Brennan wrote.

Democrats were considering countering McCain's amendment with a proposal of their own after disclosure of the administration plans. The White House wasn't expected to formally send its spending request to Capitol Hill until after the Memorial Day recess, said Kenneth Baer, spokesman for the White House Office of Management and Budget.

A military official said Tuesday that details were still being worked out on the troops' orders and destinations, adding that the timing of their deployment was not yet clear. Also undetermined was which units from which states would deploy.

The Defense Department, which has been jousting with the Homeland Security Department for the better part of a year over the possible deployment, had previously expressed concerns that the troops not be used for law enforcement duties. Pentagon officials are worried about perceptions that the U.S. was militarizing the border.

The administration's plans appear to use Guard troops only in a supporting role, according to the military official, who spoke on condition of anonymity because the details were still being worked out. Some of the troops will be armed, but others will not.

Arizona Democratic Rep. Gabrielle Giffords said the situation on the ground now is different from when Bush deployed the Guard. Arrests have fallen in the Arizona sector and there've been record drug seizures.

She said the border is more violent and law enforcement is outgunned. She and other lawmakers want the troops to be armed - they were not in the previous deployment.

She said the U.S. needs to "spend what it takes" to secure its border with Mexico.

The Mexican Embassy said Tuesday it hoped the National Guard troops would be used to fight drug cartels and not enforce immigration laws. Mexico has traditionally objected to the use of military forces to control undocumented migration, saying such measures would criminalize migrants and open the way for potential abuse.

Cecilia Munoz, White House director of intergovernmental affairs, told a group of Spanish-language reporters Tuesday that the National Guard troops would not deal directly with migrants.

More than 20,000 Border Patrol agents are deployed now, mostly along the nation's southern border.

Billeaud reported from Phoenix. Associated Press writers Andrew Taylor, Suzanne Gamboa, Lolita C. Baldor and Luis Alonso in Washington and Paul Davenport in Phoenix contributed to this report.