Economic impact

Some Democratic senators already were buoyed by Tuesday's Congressional Budget Office report, which concluded that the Senate immigration bill would reduce federal budget deficits by nearly $900 billion over 20 years and boost the US economy.

The economic projection is "a big game-changer," New Jersey Democratic Senator Robert Menendez, a member of the bi-partisan "Gang of Eight" that wrote the bill, told Reuters.

Menendez said the CBO report undermined Republican arguments that the measure would cost the government trillions of dollars over the long term, mainly in federal benefit payouts to illegal immigrants who are put on a path to citizenship.

Brian Fallon, a spokesman for Democratic Senator Charles Schumer, a member of the "Gang of Eight," said the CBO report "assumes that some immigrants who enter the country legally will overstay their visas" under new programs for temporary workers.

"The bill creates a system to track people who overstay their visas and prevents employers from hiring them, so the number is likely to be much lower than CBO projects," he said.

The Senate, controlled by Democrats, 54-46, is expected to vote on passage of the measure by the end of next week, just before Congress begins its Fourth of July holiday recess.

Menendez voiced confidence that all 54 members of the Senate Democratic caucus will support the bill, along with a number of Republicans. Backers have long hoped they could get 70 or more votes for passage in the Senate.

The bill faces an uncertain fate in the House, where Boehner declared on Tuesday that he would only bring immigration measures to the full chamber that enjoy the support of most of his fellow Republicans.

Many House Republicans have vowed to oppose a bill like the Senate's, voicing objections to the pathway to citizenship, which they argue would reward lawbreakers.