Under the emerging compromise, the government would grant legal status to immigrants living in the United States unlawfully at the same time the additional security was being put into place at a cost of tens of billions of dollars. Green cards, which signify permanent residency status, would be withheld until the security steps were complete.
Several officials said late Wednesday the Border Patrol would roughly double in size with the addition of 20,000 agents over a decade as part of a so-called border surge. The government also would complete hundreds of miles of new fencing along the U.S.-Mexico border and purchase new surveillance drones to help apprehend would-be illegal border crossers. The cost of the additional agents alone totals $30 billion over a decade.
The officials spoke on condition of anonymity because the talks were private.
If agreed to, the change has the potential to give a powerful boost to the immigration bill that is at the top of President Barack Obama's second-term domestic agenda.
The developments came as Democrats who met with House Speaker John Boehner during the day quoted him as saying he expects the House to pass its own version of an immigration bill this summer and for Congress to have a final compromise by year's end. Boehner, R-Ohio, has already said the legislation that goes to the House in the next month or two will not include a pathway to citizenship for those in the United States illegally.
Precise details of the pending agreement in the Senate were unavailable, but Sen. John Hoeven, R-N.D., said it involved a major increase of resources to the border, including more manpower, fencing and technology. 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; as well as a biometric system to track foreigners who enter and leave the United States at air and seaports and by land.
"Our whole effort has been to build a bipartisan group that will support the bill," said Hoeven, who has not yet stated a position on the legislation. "That's what this is all about, and it's focused on border security."
Sen. Chuck Schumer, D-N.Y., one of the bill's most prominent supporters, 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."
The potential compromise came into focus 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. Schumer and Sen. Bob Menendez, D-N.J., met at midday with Hoeven, and Republican Sens. Bob Corker of Tennessee, John McCain of Arizona and Lindsey Graham of South Carolina. The Democrats, McCain and Graham are part of the so-called bipartisan Gang of Eight that drafted the bill.
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 during the day on a vote of 61-37.
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.
Only a day earlier, the CBO had cheered supporters of the bill with an estimate that it would help the economy and reduce deficits in each of the next two decades.
Now it was the skeptics' turn to crow.
"Illegality will not be stopped, but it will only be reduced by 25 percent," said Sen. Jeff Sessions, R-Ala., referring to the prediction by the non-partisan CBO.
While the public debate was taking place, lawmakers involved in the private talks expressed optimism.
"We're on the verge of doing something dramatic on the border," Graham told reporters. "What we're trying to do is put in place measures that to any reasonable person would be an overwhelming effort to secure our border. This is a key moment in the effort to pass the bill."
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 faith-based Latino leaders.
"It's a conversation Republicans want to have," Rep. Cathy McMorris Rodgers, R-Wash., said later at a news conference outside the Capitol.
At the same time, though, anti-immigration protesters moved across the Capitol plaza into range of television cameras, raising signs that said, "Do Not Reward Criminals" and "No Amnesty for Illegal Aliens."
Separately, the House Judiciary Committee worked on 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.
In addition to border security measures and a pathway to citizenship for millions of immigrants in the country illegally, the Senate bill provides more visas for highly-skilled workers prized by the technology industry, a guest worker farm program and a new program for lower-skilled workers to come to the United States.