Officials said the plan envisions doubling the size of the Border Patrol with 20,000 new agents, completing 700 miles of new fencing along the border with Mexico and purchasing new surveillance drones to track would-be illegal border crossers. The cost of the additional agents alone was put at $30 billion over a decade.
In addition, immigrants would not be able to claim credit for Social Security taxes they paid while working without lawful status. Credits are used to determine the amount in Social Security benefits a worker receives after retirement.
Under another change, neither the administration nor states would be permitted to grant welfare benefits for five years to immigrants currently living unlawfully in the United States
There was no immediate reaction from the White House to terms hashed out by senators in both parties, although Democrats kept administration officials apprised of the talks.
The agreement began to take shape over the past several days beginning with a series of meetings involving Republicans who were uncommitted on the legislation but receptive to supporting it after changes were made. Eventually, Sen. Chuck Schumer, D-N.Y., and Sen. Bob Menendez, D-N.J., both authors of the bill, joined the talks.
If agreed to, the changes could clear the way for a strong bipartisan vote within a few days to pass the measure that sits atop President Barack Obama's second-term domestic agenda.
The officials who described the emerging deal spoke on the condition of anonymity because the talks were private.
The developments came as Democrats who met with House Speaker John Boehner on Wednesday quoted him as saying he expected the House to pass its own version of an immigration bill this summer and Congress to have a final compromise by year's end.
Boehner, R-Ohio, already has said the legislation that goes to the House in the next month or two will not include a pathway to citizenship for immigrants in the United States illegally.
The potential compromise in the Senate came into focus Wednesday, one day after the Congressional Budget Office jolted lawmakers with an estimate saying that as drafted, the legislation would fail to prevent a steady increase in the future in the number of residents living in the United States illegally.
The estimate appeared to give added credibility to Republicans who have been pressing Democrats to toughen the border security provisions already written into the bill.
"Our whole effort has been to build a bipartisan group that will support the bill," said Sen. John Hoeven, R-N.D., who's helped develop the deal along with Sen. Bob Corker, R-Tenn. "That's what this is all about, and it's focused on border security."
Schumer and Menendez met at midday Wednesday with Graham, Hoeven, Corker and Sen. John McCain, R-Ariz. The Democrats, Graham and McCain are part of the so-called Gang of Eight.
It was unclear which other portions of the Senate legislation might be changed. There is pressure from some Republicans to make sure no federal benefits go to immigrants who are in the country illegally, at least until they become citizens.
The underlying legislation already envisions more border agents; additional fencing along the U.S-Mexico border; surveillance drones; a requirement for employers to verify the legal status of potential workers; and a biometric system to track foreigners who enter and leave the United States at air and seaports and by land.
Schumer said discussions with Republicans "have been really productive."
"We've made a lot of progress in the last 24 hours. Now we have some vetting to do with our respective allies," he said.
If ratified, the compromise would mark concessions on both sides.
Some Republicans have been unwilling to support a bill that grants legal status to immigrants in the country illegally until the government certifies that the border security steps have achieved 90 percent effectiveness in stopping would-be border crossers.
On the other hand, Democrats have opposed Republican proposals to make legalization contingent on success in closing the border to illegal crossings. Under the legislation as drafted, legalization could begin as soon as a security plan was drafted, but a 10-year wait is required for a green card.
One plan to change that was sidetracked on a vote of 61-37 Wednesday.
Sen. Rand Paul, R-Ky., said his proposal would require Congress to vote annually for five years on whether the border is secure. If lawmakers decide it is not, "then the processing of undocumented workers stops until" it is, he said. The decision would be made based on numerous factors, including progress toward completion of a double-layered fence along the U.S.-Mexico border and toward a goal of 95 percent capture of illegal entrants. A system to track the border comings and goings of foreigners is also required.
Across the Capitol, House Republican leaders sought to present a friendlier face to Hispanics a group that gave Obama more than 70 percent support in last year's presidential election.
Boehner met with the Democratic-dominated Congressional Hispanic Caucus, while rank-and-file members of his party reviewed areas of agreement with Latino religious leaders.
"It's a conversation Republicans want to have," Rep. Cathy McMorris Rodgers, R-Wash., said later at a news conference outside the Capitol.
Separately, the House Judiciary Committee approved legislation creating a program allowing farm workers to come to the United States to take temporary jobs in the United States.
The measure is one of several that the panel is considering in the final weeks of June as part of a piece-by-piece approach to immigration rather than the all-in-one bill that Senate is considering.
Associated Press writer Laurie Kellman contributed to this report.