An administration official confirmed to Fox News that Obama plans to deploy the National Guard troops as needed and request $500 million for "enhanced border protection and law enforcement."
The official said the National Guard would be used to "provide intelligence, surveillance and reconnaissance support," as well as support "counternarcotics enforcement" and provide "training capacity" until the Border Patrol can bring more officers on board. The additional funding would be used to improve border security technology and increase the number of agents, investigators and prosecutors targeting drug, human and weapons traffickers.
Sen. Jon Kyl, R-Ariz., said he's heard that "the 1,200 border patrol troops are, in effect, desk jobs."
"They aren't boots on the ground at the border," he said, adding that "they were not intended to be deployed to the border."
"Rather they'll be investigating, administrative support, maybe training," he said. "Now that's all fine...but the real value of the National Guard is to be seen."
Arizona Gov. Jan Brewer said Obama's plan represents a "very significant and important shift in the president's immigration and border security policy."
"I am pleased that President Obama has now, apparently, agreed that our nation must secure the border to address rampant border violence and illegal immigration without other pre-conditions, such as passage of 'comprehensive immigration reform,'" she said.
"I am anxious to hear of the details that have not yet been disclosed of where, how, and for how long additional forces will be deployed," she added. "With the accountability of this election year, I am pleased and grateful that at long last there has been a partial response from the Obama administration to my demands that Washington do its job."
Rep. Gabrielle Giffords, D-Ariz., praised the administration for "finally" answering the calls for more troops, while another lawmaker said 1,200 troops is "simply not enough."
The announcement came as Senate Republicans began introducing several border security amendments to a $60 billion war spending bill under consideration on the floor. They showed no signs of halting that effort in light of the president's decision.
Sen. John McCain, R-Ariz., proposed an amendment to send 6,000 National Guard troops to the border -- a move Republicans want to pay for with unspent stimulus money. McCain said on the floor that he appreciates Obama's decision to deploy up to 1,200 troops but asked for more.
"I think it is a recognition of the violence on the border which has been really beyond description in some respects," McCain said. "But it's simply not enough."
Senate Republicans confronted Obama on immigration and border security earlier in the day during a rare, private meeting on Capitol Hill. But Obama never mentioned he was sending troops to the border, Kyl said, even after he brought up the McCain amendment.
"I gather that some of our Democratic colleagues were informed," Kyl said. "Sen. McCain spoke to it...and then I stood up. One of the things I said was we were going to the floor in a few minutes to request additional money for sending troops to the border. But that was the end of the conversation."
Though top Obama Cabinet officials critical of Arizona's controversial new law have acknowledged they never read the bill, McCain said Obama told the senators he has read it and still believes it could allow for discrimination.
Sen. Sam Brownback, R-Kansas, who described the meeting as "testy," said the president "wasn't embracing" the call to secure the borders before pressing forward with a comprehensive immigration policy overhaul.
The administration official described Obama's announcement Tuesday as "part of his comprehensive plan to secure the southwest border."
Giffords, who was among several lawmakers who requested more border security after Arizona rancher Robert Krentz was killed by a suspected illegal immigrant at the end of March, praised Obama for the National Guard announcement.
"(Arizona residents) know that more boots on the ground means a safer and more secure border. Washington heard our message," she said.
Fox News' Trish Turner contributed to this report.