A U.S. Senate compromise on an overhaul of immigration law appeared to get bogged down late on Thursday in the face of opposition by some Republicans who say it would give amnesty to lawbreakers.

It appeared unlikely the Senate would approve before a two-week congressional break on Friday the radical immigration reform that would give millions of illegal immigrants a chance to earn U.S. citizenship.

The bill's backers expressed fear that failure to enact the legislation before lawmakers leave for their spring break could hurt its chances in a congressional election year.

Senate Republican Leader Bill Frist of Tennessee and Senate Democratic Leader Harry Reid of Nevada earlier hailed a "breakthrough" on the compromise bill that would include a guest worker program sought by President George W. Bush.

But the two leaders were unable to agree on how many amendments to the bill would be allowed.

Frist later accused Democrats of trying to block the bill by refusing to allow amendments. Democrats said they wanted to protect the bill from being "gutted" by Republicans opposed the bill.

"Republicans are still deeply divided on this issue, and we must protect this from those senators bent on gutting the bill with hostile amendments," said Reid.

The congressional debate over how to deal with an estimated 11 million illegal immigrants has sparked large protests in many U.S. cities against a crackdown and more demonstrations are planned in 10 cities on Monday.

The Senate is considering a compromise that would allow illegal immigrants who have been in the United States more than five years, about seven to eight million people, a chance to become citizens if they pay a fine, pay their taxes, and meet other requirements such as learning English.

BUSH CALLS FOR QUICK PASSAGE

Bush said earlier he was encouraged by the compromise and urged quick passage of the legislation.

The Mexican government in a statement called the agreement "an important step toward to establishing new mechanisms that permit legal, safe and ordered migration which respects human rights."

Any Senate bill still has to be merged with a House of Representatives version that focuses on border security and enforcement and makes it a felony, instead of a civil offense, to be in the country illegally. It also calls for constructing a fence along parts of the U.S. border with Mexico.

A number of conservative House Republicans also oppose Senate provisions that would give illegal immigrants a chance at legal status and citizenship.

"I do not believe a plan of this nature can pass the House," said Rep. Thomas Tancredo (news, bio, voting record), a Colorado Republican.

The compromise also drew opposition from the AFL-CIO labor federation which said it would drive down wages for all workers and fail to treat all illegal immigrants equally.

The Senate bill would allow some three million illegal immigrants who had been in the United States less than five years but more than two years to apply for a work visa.

But they would have to travel to a U.S. port of entry to get it.

Illegal immigrants who had lived in the country less than two years, roughly 1.5 million, would have to leave to apply for a visa to re-enter.

(Additional reporting by Adriana Barrera in Mexico City, Joanne Kenen, Vicki Allen, Thomas Ferraro in Washington)