Washington (CNN) -- A year and a half after deploying National Guard troops to counter illegal immigration on the Southwest border, the Department of Homeland Security and Pentagon told Congress Tuesday they are cutting the number of troops and changing their mission.
The drawdown, which the department characterized as a "transition," will begin in January and should be completed by March. Several lawmakers told CNN the number of National Guard troops on the border will be cut from 1,200 with responsibilities mainly on the ground to 300 who will support the border mission in the air.
The Department of Homeland Security said the change is possible because of a jump in the number of Border Patrol officers in the region, an increase in technology and a drop in apprehensions at the border.
But several lawmakers criticized the move, saying conditions on the Southwest border merit more troops, not fewer.
A deployment of 1,200 troops was "not enough to begin with, although they are doing as good a job as they can," Rep. Ted Poe, R-Texas, told CNN. Poe said the United States has "operational control" of only 44 percent of the Southwestern border.
"That's not success," Poe said.
Rep. Lamar Smith, R-Texas, agreed.
"If the Obama administration's goal is border security, their actions undermine their objective," he said in a written statement. "We should keep guardsmen on the ground until the Border Patrol can gain operational control of the majority of the U.S.-Mexico border."
But Rep. Silvestre Reyes, D-Texas, supported the drawdown. "While I appreciate the service of our National Guard forces, requiring them to engage in border law enforcement activity is not cost effective," he said in a statement.
The Homeland Security Department said that in the fiscal year ending October 1, Border Patrol apprehensions of illegal immigrants -- a key indicator of the amount of illegal immigration -- decreased to 340,252, down 53 percent since fiscal year 2008. Since 2004, the size of the Border Patrol has doubled to 21,444.
In a news release announcing the change, the Department of Homeland Security and the Pentagon did not cite exactly how many troops would be pulled from the border, but said the "new strategic approach" will increase border security. That approach includes "adding a number of new multi-purpose aerial assets" equipped with the "latest surveillance and reconnaissance capabilities."
In addition, the National Guard will provide helicopters and planes to the Border Patrol, "allowing for faster response capabilities," the news release said.
"I did not understand or get the impression they are going to add any equipment; they're going to use what they have," said Poe, who was briefed on the changes.
The Defense Department budget for the border would drop by half, from $120 million to $60 million, congressional staffers said they were told. Specifics about the ramped-up aerial surveillance are still being worked out, the staffers said.
Poe said he plans to fight the change. "I'll ask the president to reconsider and then try to figure a way, bipartisan, to show that this is not a wise move," he said.