WASHINGTON (AP) — The Senate headed Monday for a crucial test vote on White House-backed immigration legislation offering the prize of U.S. citizenship to millions and pouring new technology and manpower into the border.

Ahead of the vote set for early evening, around a dozen Republican lawmakers had indicated support, setting up a solid bipartisan margin of victory within reach of the 70 votes supporters are hoping for when the bill comes to a final vote at the end of this week. No defections have been suggested so far among the 54 votes controlled by Democrats.

The measure includes changes to the original border security provisions in the bill that would double the size of the U.S. Border Patrol at a cost of around $30 billion and complete 700 miles of fencing. At the same time it sets out a pathway to citizenship for some 11 million immigrants living in the United States illegally, who would be permitted to get permanent resident green cards only once all the border changes had been put in place, about a decade after enactment of the legislation.

At the White House Monday, President Barack Obama called on lawmakers to take quick action on the immigration overhaul.

"I would urge that the Senate bring this to the floor," he said. "And I hope that we can get the strongest possible vote out of the Senate so that we can then move to the House and get this done before the summer break."

The president spoke in the Roosevelt Room before meeting with business leaders supporting the immigration bill. He said the legislation doesn't represent everything he sought, "but it does adhere to the core principles that we need for comprehensive immigration reform."

"Now is the time to do it," he added.

Meanwhile, on the Senate floor, supporters and opponents traded arguments ahead of the vote.

"It's my hope that this evening Republicans will join me in putting in place the toughest border security measures we've ever had in this nation," said Sen. Bob Corker, R-Tenn., who developed the new border security requirements with Sen. John Hoeven, R-N.D.

Sen. Jeff Sessions, R-Ala., a leading opponent, insisted that the promised border security never would materialize.

"The amnesty occurs first, and just like so often in the past, the promises never occur," Sessions said.

But Corker and other supporters pointed to comments from Arizona Gov. Jan Brewer, a Republican immigration hardliner, who touted the proposed border changes as a "victory for Arizona" in an interview on Fox News Channel Monday.

The developments came at the start of a crucial week for the immigration bill, a signature issue for the Obama administration, Capitol Hill Democrats, and even some Republicans.

Monday's vote is the key procedural hurdle that would clear the way for a vote later in the week on revisions to the bill including the border security changes and a range of other new provisions aimed at locking down support from wavering senators. These include limits to what newly legalized immigrants can claim in Social Security benefits, added at the behest of Sen. Orrin Hatch, R-Utah, and provisions designed to aid Alaska seafood processers and attract support from Alaska's two senators.

Final passage of the underlying bill should come by Friday. In addition to securing the border and allowing eventual citizenship to those here illegally, the bill would create new visa programs to bring tens of thousands of high- and low-skilled workers to the country, and require all employers to check their workers' legal status. In many states such employer checks are now only voluntary.

The U.S. Chamber of Commerce announced it was launching a new seven-figure ad buy Monday in support of the bill. "Call Congress. End de facto amnesty. Create jobs and economic growth by supporting conservative immigration reforms," the ad said.

On the other side, a political committee run by tea party Republican Sen. Ted Cruz of Texas issued an action alert calling on supporters to lobby senators against the bill. "It will encourage more illegal immigration and must be stopped," read his appeal.

Victory in the Senate would be no guarantee of success in the Republican-controlled House, where many conservatives oppose citizenship for people in this country illegally. The House Judiciary Committee has been passing narrowly focused, single-issue immigration bills — in contrast to the Senate's comprehensive approach — and Speaker House Boehner, R-Ohio, has not indicated how he'll proceed.

Negotiations between the two chambers are not expected until the fall at earliest, and opponents of the legislation are predicting it will be stopped in the House.

"It will pass the Senate, but it's dead on arrival in the House," Sen. Rand Paul, R-Ky., said on CNN on Sunday. "The House is much closer to me, and I think they think border security has to come first before you get immigration reform."