WASHINGTON — As Congress begins debate on a complex immigration bill, the leading Republican candidates for president have finally found a policy issue to squabble over.

The chief differences have emerged between Senator McCain of Arizona, who helped forge a fragile agreement with Senator Kennedy in the Senate, and Mitt Romney, who quickly came out against the bill and derided it as a form of “amnesty” for illegal immigrants. In the middle is Mayor Giuliani, who has joined many politicians on both sides of the aisle in taking a wait-and-see approach to the bill, which may face a slew of amendments in the Senate.

Such a clear distinction on a policy matter has been rare among the top three Republican hopefuls, with the exception of social issues such as abortion, on which Mr. Giuliani has parted ways with his rivals. On most other issues, they have sounded largely similar themes. On Iraq, all three oppose a withdrawal of American troops, and they have all pledged to fight the war on terrorism aggressively. Each has vouched for similar versions of fiscal conservatism, championing lower taxes and promising to reduce government spending.

On the surface, the immigration issue is fraught with risk for Mr. McCain while providing an opening for Mr. Romney to align himself with the Republican base. Like the former Massachusetts governor, many conservatives have railed against measures in the bill that would give probationary legal status, through a “Z-visa,” to the estimated 12 million illegal immigrants in the country and would provide many of them a path, though long and expensive, to full citizenship.

“Any legislation that allows illegal immigrants to stay in the country indefinitely, as the new ‘Z-visa’ does, is a form of amnesty,” Mr. Romney said last week.

The visa would apply to illegal immigrants who can prove that they entered the country before January 1, 2007. Immigrants would be subject to a background check, and the visa would be renewable after four years. Ultimately, immigrants would have to pay $5,000 in fines before becoming eligible for permanent residency status and citizenship, and the head of household first would have to return to the family’s home country.

While Mr. Romney’s stance may appeal to conservatives, he is likely to face criticism that on immigration, as with other issues, his position has changed over the years. In November 2005, he told the Boston Globe that immigration proposals by Mr. McCain and others were “quite different than amnesty” and that the legislation that the Arizona senator introduced with Mr. Kennedy was “reasonable.” He has since come out against the bill that passed the chamber late last year, and he now opposes a path to citizenship for illegal immigrants.

A campaign spokesman, Kevin Madden, said yesterday that Mr. Romney had been discussing an early version of the bill, which went through numerous changes in committee in later months.

The McCain campaign has yet to hit back at Mr. Romney publicly on immigration, but a spokesman yesterday hinted at what the strategy might be. “Senator McCain is a leader that tackles tough issues and is seeking the presidency to put forward serious solutions, not playing politics with the challenges confronting our nation,” the spokesman, Danny Diaz, said.

The bill faces an uphill climb in Congress, as both Republicans and Democrats have raised objections to various provisions. But if it passes, the legislation would surely boost Mr. McCain’s reputation for bipartisanship, a point he has emphasized in recent weeks. “It’s probably good for him if he’s the Republican nominee, because it shows he can effectively work across the aisle,” the dean of Baruch College’s School of Public Affairs, David Birdsell, said.

It will make the Republican primary an even tougher challenge, however. None of the other nine declared candidates has expressed support for the bill, and the issue could give new prominence to a Colorado congressman, Tom Tancredo, who is running an explicitly anti-immigration campaign.

For Mr. Giuliani, the strategy may be to let the other candidates fight over immigration among themselves. In Florida on Friday, he said that if a new system provided for a “tamper-proof ID card” and a database to monitor who is in the country — both are part of the proposed bill — there could be “a lot of flexibility” to achieve a compromise.