By Richard Cowan and Thomas Ferraro
WASHINGTON (Reuters) - Federal agents on the U.S.-Mexican border would double to about 40,000 under a deal reached on Thursday in the Democratic-led Senate to draw more Republicans to a landmark immigration bill headed toward anticipated passage.
Some questioned the costs and benefits of up to $50 billion in the extra border security, which also will include high-tech surveillance equipment such as manned and unmanned aerial vehicles, radar and seismic devices.
But concerns were overshadowed by the deal's main goal: win votes for an overhaul of U.S. immigration law that will open a pathway to citizenship for up to 11 million undocumented immigrants.
Senate Democratic leader Harry Reid was expected to set a test vote for as early as Monday in a bid to have the deal added to the White House-backed bill in the form of an amendment.
A senior Democratic aide predicted the amendment would get upward of 60 votes in the 100-member chamber, more than enough to clear any procedural roadblocks.
A vote on passage of the bill is expected before the Senate departs at the end of next week for its Fourth of July holiday recess.
Backers are aiming for at least 70 votes on passage to increase pressure on the more resistant Republican-led House of Representatives to give the bill final congressional approval.
Republican John McCain, a member of the "Gang of Eight" senators who wrote the bipartisan bill, voiced doubt about the high cost of additional border security.
"I don't know if it's totally well spent," he said.
But McCain added, "I think it's important that we do this to give people confidence that we have border security, so in that respect it's well spent."
A leading conservative voice embraced the deal.
Republican Senator Marco Rubio, a Cuban-American lawmaker from Florida and another member of the Gang of Eight, said the deal was a "dramatic improvement in border security" during an interview on Fox News.
Rubio, touted as a possible 2016 presidential candidate, had hinged his support on improvements in border security. His endorsement is seen as crucial to winning conservative backing for the biggest changes in U.S. immigration law in a generation.
The proposal would double the overall number of U.S. border patrol agents, according to senior Senate Democratic aides.
That would mean assigning 21,000 new officers to the border with Mexico in an attempt to shut down illegal crossings by foreigners.
"I am now confident ... that the Senate will pass a strong, bipartisan immigration reform bill and that it will ultimately reach the desk of the president for his signature," Democratic Senator Charles Schumer of New York said.
The immigration bill, which is supported by President Barack Obama, currently calls for adding 3,500 Customs and Border Protection officers by 2017.
Besides doubling the number of border agents, the deal also calls for completing the construction of 700 miles of border fencing or walls, Senate aides said. About 650 miles have been built in one form or another, although some portions will have to be upgraded.
At an estimated price tag of about $40 billion to $50 billon, the amendment would represent a potentially massive investment of federal resources in securing the border at a time when conservatives are complaining about government outlays.
As originally written, the legislation called for about $6 billion in new border security spending.
CRITICS STILL UNHAPPY
Republican Senator Jeff Sessions of Alabama mocked the deal even though he has called for tougher border enforcement. He noted that it was drafted after congressional analysts estimated the bill would trim illegal immigration by just 25 percent.
"The bill gets in trouble on the floor and they scurry around to get an amendment to throw 20,000 agents ... somewhere on the border in the future, we promise." Sessions said, adding that such promises have been made in the past but not honored.
Sessions and other conservatives have pushed for delaying any pathway to citizenship for 11 million people until the government virtually eliminates illegal border crossings.
But the Senate repeatedly has repelled such attempts. On Thursday, it voted 54-43 to kill an amendment by Senator John Cornyn of Texas, the second-ranking Republican, which would have delayed permanent legal status for undocumented immigrants until the government met strict border enforcement goals.
On Tuesday, the Congressional Budget Office estimated that the Senate's immigration bill would save the federal government nearly $900 billion over 20 years as illegal immigrants became legal, tax-paying residents.
A Democratic aide said those projected savings gave senators the leeway to craft such an expensive border security amendment.
House Speaker John Boehner, an Ohio Republican who has promised to consider an immigration bill this year, told reporters the CBO deficit-reduction estimates, if "anywhere close to being accurate, would be a real boon for the country."
While the legislation authorizes the beefed-up security programs, it would be up to Congress in the future to appropriate the funding.
A Senate aide said the newly legalized residents would not get "green cards" allowing permanent resident status until the border security measures were in place.
Gaining permanent resident status would take 10 years under the bill, giving the federal government the time to deploy the added border officers and equipment.
(Additional reporting by Rachelle Younglai and David Lawder; Editing by Fred Barbash and Bill Trott)