President Barack Obama on Tuesday ordered 1,200 National Guard troops to boost security along the U.S.-Mexico border, pre-empting Republican plans to try to force votes on such a deployment.

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

The president announced the deployment shortly after he returned from lunch with the 41-member Senate Republican Caucus, who told him that U.S. borders first need to be secured before work could start on a sweeping overhaul of U.S. immigration laws. Arizona's two Republican senators said the deployment wasn't enough.

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.

Administration officials said 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.

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.

Officials also said the funds will be used to enhance technology at the border and share information and support between law enforcement agencies as they target illegal trafficking in people, drugs, weapons and money.

More than 20,000 Border Patrol agents are deployed now, mostly along the nation's southern border. There are 344 U.S. National Guard troops also working along the border.

Immigration issue in spotlight

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.

Arizona Gov. Jan Brewer on Tuesday credited her signing of the controversial new law for compelling Obama to act. The law requires state and local police to investigate the immigration status of people they reasonably suspect are in the country illegally. Brewer said in a statement that the law "clearly ignited the talk of action in Washington for the people of Arizona and other border states."

Brewer echoed other officials and said she was "anxious" to hear additional details setting out where, how, and for how long additional forces would be deployed.

After the lunch with Obama, Sen. John McCain, R-Ariz., described the immigration discussions. "We had extended conversation. We didn't agree. Border security first."

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 and fellow Arizona Republican Sen. Jon Kyl also issued a joint statement: "The fact that President Obama announced today that he will only be sending one-fifth of the troops we believe are required is a weak start and does not demonstrate an understanding of the current situation in the region."

McCain's amendment came under scathing attack in the letter from Jones and Brennan. "It represents an unwarranted interference with the commander in chief's responsibilities to direct the employment of our armed forces," they wrote.

Still, McCain's overall view found agreement with at least one Arizona Democrat.

"Adding 1,200 National Guard troops is a start, but it is going to take much more to secure the border," said Rep. Harry Mitchell.

A prominent cattleman, Robert Krentz, was shot dead on his ranch in southeast Arizona in late March. Police followed tracks from the scene of the shooting to the Mexican border, but made no arrests.

Cochise County sheriff Larry Dever, whose office is investigating Krentz's death, said he doubted that many troops would make much difference, given the border's nearly 2,000-mile (3200-km) length.

"If you put 1,200 in perspective ... that's about one every 2 miles ... so we're woefully short of doing anything significant, unless they are all deployed in a very specific area," he said.

Militarizing the border

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.

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 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.