Reid's move raises the political stakes for Republicans, including presidential aspirant John McCain, who helped Democratic colleague Edward Kennedy to drive last year's bill to passage.

But McCain has since shied away from its expansive proposals to provide legal status for the estimated 12 million people in the country illegally and open the door to more temporary foreign workers in the years ahead.

The Arizona senator, mindful that the conservative Republicans who will be crucial in next year's primaries tend to favor more border security and tighter immigration controls, has adopted a more cautious stance. He has joined other Republicans in urging Reid to allow private, bipartisan talks to continue rather than picking up where the Senate ended last year.

“He wants a bill that is bipartisan and that ensures that border security is first,” McCain spokeswoman Melissa Shuffield said yesterday.

The latest congressional immigration drama is unfolding against a backdrop of frustration on all sides.

Immigration advocates across the nation have taken to the streets demanding that immigrants' work be rewarded with green cards and a path to citizenship. Opponents have held rallies warning that such a step would fuel illegal entry by showing that the United States is too divided to enforce its own immigration laws.

Congress is groping for a way to fix a system universally regarded as broken.

Any potential solution will have to address major questions: How many immigrants will be allowed? Who will benefit and who will lose under the changes?

“One issue is that the benefits come mostly to interest groups, while the costs are widely diffused” throughout society, said Michael Teitelbaum, a demographer with the nonprofit Alfred E. Sloan Foundation.

Frank Sharry of the National Immigration Forum, which advocates for expansive immigration policies, says federal policy should reflect the reality that employers depend on low-wage immigrant labor.

“The best way to serve the national interest is to allow people to come legally within realistic limits and to have a process by which people already here working and contributing can legalize their status,” Sharry said.

For the past two months, the Bush administration has been attempting to broker a deal that would avoid last year's stalemate, when the Senate passed its sweeping legalization bill and the House focused on tightening the nation's borders.

Reform advocates are urging Congress to move quickly, warning that preparations for the 2008 presidential election and other pressing business might make it impossible for lawmakers to concentrate on immigration after the fall.

Even in the Senate, the going has been rough because of differences between the pro-immigration ranks, led by Kennedy of Massachusetts, and the restrictionists, led by Republican Jon Kyl of Arizona. In a major concession, Kyl has indicated a willingness to consider legalization for the illegal immigrants now in the country.

But in return, Kyl demands that future temporary workers be required to go home after a fixed period. Kennedy wants them to have a guaranteed chance at a green card, which would put them on a path to citizenship.

Their divisions reflect the gap in perceptions of how the current wave of immigrants fits into the historical narrative of immigration. The fundamental question: Will today's immigrants be able to repeat the upward climb achieved by previous waves?

Kennedy is optimistic. “The values that immigrants bring, of hard work, family and faith, are values that strengthen our nation enormously,” he said.

Kennedy says the children of today's illegal immigrants will find the middle-class success that has long defined the American dream.

But Kyl worries that the rising numbers of immigrants at the lowest rungs of the economy will harm citizens competing for the same jobs. He also worries that low-skilled immigrants could form a permanent underclass.

Some Republicans also want to overhaul the green-card system, which gives an enormous advantage to people related to U.S. citizens and green-card holders. While they believe that immediate family members should continue to be eligible for green cards, they do not think adult children and siblings should get them based on family ties.

They want more visas for immigrants with specialized skills and abilities. Their proposal, floated in a White House draft plan in March, brought an immediate reaction from the Asian-Americans and Latinos who have been the principal beneficiaries of the family-based system.

“This would undermine Asian-American communities,” said Karen Narasaki, president of the Asian American Justice Center.

GOP strategists fear that if Republicans try to keep Reid from getting the 60 votes he would need next week to open the debate with last year's bill, Democratic ad campaigns would brand them as anti-immigrant.

Meanwhile, House Speaker Nancy Pelosi, hoping to head off possible Republican campaign spots denouncing a “Kennedy-Pelosi amnesty,” has told the White House it will have to deliver at least 70 GOP votes to pass any bill that comes to the House floor.

There is broad feeling in Congress that the public expects action to manage a problem that has become what even longtime immigration advocate Rep. Howard Berman, D-Los Angeles, calls “a national crisis.”

Much of the urgency comes from a sense of unease that has swept across the country in the past decade as illegal immigration – once an issue in a handful of states – has surged in such heartland regions as Iowa, Indiana and Georgia. The country's illegal-immigrant population is now growing at about 500,000 per year, said demographer Jeffrey Passell of the Pew Hispanic Center. That's part of a steady climb, from annual rates of about 65,000 in the mid-1980s and 320,000 in the mid-'90s.

The trend has been fueled by Congress, which passed a series of measures that have boosted legal immigration from 265,000 in 1960, to 373,000 in 1970, to 524,000 in 1980, to 1.26 million last year. Much of the recent increase can be traced to a 1986 amnesty that gave green cards to 2.7 million illegal immigrants, who soon began sponsoring relatives for green cards.

Kennedy and others are pushing for a legalization program that would dwarf the 1986 amnesty. Their plan would make newly legalized immigrants eligible to bring in relatives, and the number of immigrants could soar above today's levels. Opponents say such growth, especially if it continues to draw from low-skilled, poorly educated immigrants, would become an enormous burden on taxpayers.