About 8 million immigrants living unlawfully in the United States would initially gain legal status under sweeping legislation moving toward a vote in the Senate, the Congressional Budget Office said Tuesday, adding the bill would push federal deficits lower in each of the next two decades.

The eagerly awaited report by Congress' non-partisan scorekeeping agency said the legislation would increase federal spending in the form of benefits for those gaining legal status, but those expenses would be more than offset by a rise in the labor force, increasing revenues.

Supporters of the legislation said the report would add to the momentum behind a measure that toughens border security at the same time it holds out the hope of citizenship to millions who came to the United States illegally or overstayed their visas.

The CBO said deficits would fall by $197 billion across a decade, and by $700 billion in the following 10 years if the bill became law. The assessment came as the pace of activity increased at both ends of the Capitol on an issue that President Barack Obama has placed at the top of his domestic agenda.

Challenged by protesters chanting "shame, shame," House Republicans advanced legislation to crack down on immigrants living illegally in the United States, at the same time the Senate lurched ahead on a dramatically different approach offering the hope of citizenship to the same millions.

Republican Rep. Trey Gowdy of South Carolina said the bill moving through the House Judiciary Committee was part of a "step by step, increment by increment" approach to immigration, an issue that can pit Republican against Republican as much if not more than it divides the two political parties.

California Democratic Rep. Zoe Lofgren predicted there would be "millions of American citizens taking to the street" in protest if Republicans pressed ahead with the bill. The measure permits state and local authorities to enforce federal immigration laws and requires mandatory detention for anyone in the country illegally who is convicted of drunk driving.

Despite the protests, approval by the committee was a foregone conclusion. The panel's chairman, Rep. Robert Goodlatte, R-Va., said future bills would require companies to make sure their employees are living in the United States legally, create a program for foreign farm workers who labor in the United States and enhance the ability of American firms to hire highly skilled workers from overseas.

Those steps and more are already rolled into one sweeping measure in the Senate, a bipartisan bill that Obama supports and that appears on track for a final Senate vote as early as July 4.

In a series of votes during the day, the Senate rejected a move by Sen. John Thune, R-S.D., to require the installation of 350 miles of fencing along the U.S.-Mexico border before legalization can begin for anyone currently in the United States illegally.

Similarly, the Senate rejected a proposal by Sen. David Vitter, R-La., to prevent legalization until a biometric system is in place to track people entering or leaving the country through air, sea or land points of departure.

Those proposals were overshadowed by a larger debate over the types of border security requirements the legislation should contain. Republicans generally want to toughen the existing measure, particularly since the bill includes a 13-year path to citizenship for immigrants in the country illegally Γ(euro) " a provision that sparks opposition from voters who could be influential in GOP primaries in next year's mid-term elections.

Sen. John Hoeven, R-N.D., told reporters that he and others want the government to demonstrate an ability to apprehend the vast majority of those attempting to enter the country illegally before anyone already present can take the first step toward possible citizenship.

Democrats have previously been unwilling to consider proposals along those lines, arguing they could postpone legalization for years if not longer. As drafted, the bill gives the government six months to develop a plan to achieve border security, but does not hold up legalization while it is being tested for effectiveness.

It was unclear what, if any, compromise is possible on that point. Agreement would greatly increase the bill's chances for passage with a large bipartisan vote.

The measure was drafted by a bipartisan Gang of Eight and represents a series of political trade-offs among senators as well as outside groups like business and labor, growers and farm workers. In addition to border security and a path to citizenship, it includes an expanded number of visas for highly skilled workers prized by the technology industry and a new program for low-skilled workers. It also features a top-to-bottom overhaul of a decades-old system for parceling out visas to future legal immigrants, reducing the importance of family ties while emphasizing education, job skills and youth.

Broad in its scope, the bill calls for new judges, prosecutors and other officials to handle cases involving immigration law. At the same time, it would require the government to pay for legal representation, if necessary, for unaccompanied children caught up in such cases, as well as for adults determined to be legally incompetent because of a serious mental disability.

Any talk of compromise in the House appeared distant as Speaker John Boehner, R-Ohio, sought to reassure conservatives who have expressed fears he will allow legislation to come up this summer that they oppose and Democrats support.

One official who attended the closed-door meeting quoted the Ohio Republican as saying he has no intention of allowing a bill to come up that would violate the principles of the GOP majority and split its ranks. The speaker also made clear that legislation must satisfy Republican concerns about border security, according to the official.

At roughly the same time, Goodlatte gaveled the Judiciary Committee to order, and more than a dozen protesters who had been seated in the hearing room stood up and began clapping and chanting, "Shame, shame, shame! More of the same!" They were ushered out but their cries could still be heard in the hallway and committee proceedings were briefly interrupted.

The bill was the first on immigration to move through the committee this year, but hardly the opening salvo from conservatives who hold sway in the House.

Two weeks ago, the House voted to overturn Obama's 2012 election year announcement that he was suspending deportation of many immigrants who came to the country illegally as children.