Immigration reform continues to split the Senate as no single plan to address the more than 11 million illegal immigrants in the United States has emerged with enough support to defeat a deal-ending filibuster.

Key votes testing the viability of any of the proposals being debated could occur on Thursday. The so-called cloture votes would shut down any possible filibuster. It would take 60 senators to defeat the parliamentary tactic, but at this point no bill has the support of 60 senators.

"I do not think we have the votes for cloture right now or it's very close, and to have a cloture vote and fail would just set us back," said Senate Judiciary Committee Chairman Arlen Specter, R-Pa., who helped usher to the Senate floor the most popular bill, sponsored by Sens. John McCain, R-Ariz., and Edward Kennedy, D-Mass.

McCain said a majority in the 100-member Senate support his and Kennedy's proposal to provide green cards to illegal immigrants after they've worked in the United States for six years. But the lawmakers acknowledge that conservatives who oppose the measure on grounds it amounts to amnesty have enough support to block the measure.

Immigration has sharply divided Republican senators as well as the rest of the nation. Hundreds of thousands of immigrant protesters have marched in cities around the country in support of a guest worker program, and organizers are planning a "National Day of Action" on April 10 in 65 cities in support of a guest worker plan.

On the flip side, volunteer groups like the Minuteman Project have assailed the Bush administration for lax border security and started going out on their own to monitor the border and report border jumpers to federal officials. Opponents say President Bush's support for a guest worker program amounts to "amnesty," or giving illegal immigrants greater access to citizenship than those who enter according to current U.S. law.

"What we cannot support ... is amnesty. To me, amnesty is when you give someone who has clearly broken the law a leg up on the pathway to citizenship. Giving illegal immigrants a special path to citizenship essentially rewards people who broke the law," said Senate Majority Leader Bill Frist, R-Tenn.

Frist has offered a separate bill that does not deal with illegal immigrants, but boosts border enforcement and cracks down on employers who hire illegal workers. The House in December passed a bill that would make being in the country illegally a felony.

Frist spoke after the vast majority of the Senate GOP attended an hour-long meeting Tuesday morning to hear proposals for a compromise. But numerous members emerged to say no compromise had been found.

What had been offered at the meeting was a plan by Sens. Mel Martinez of Florida and Chuck Hagel of Nebraska, in what is being dubbed a "roots concept" bill.

The Martinez-Hagel bill would propose splitting illegal immigrants into two categories: those who have been in the United States for more than five years and those who are newer arrivals. Those who can prove they have been in the United States for at least five years would be allowed to stay and file for legal residency and citizenship under the rules set out by the McCain-Kennedy plan. According to that plan, illegals would have to pay a $2,000 fine and back taxes, would have to be proficient in English and civics and would have to pass a background check. After 11 years, they could become U.S. citizens.

Those who have been in the United States less than five years would have to go to "a point of entry" like El Paso, Texas, and fill out papers to stay. They would not have to return to their country of origin. It's unclear if these people would be able to get on the path to citizenship; most likely they would not.

"I think I see the emergence of a Republican position on this issue, a compromise," Martinez told FOX News, though no other lawmakers or aides questioned agreed with that assessment.

Several members predicted fraud would be rampant under the Martinez-Hagel proposal because it would be difficult for illegals to prove they've been in the county for five years or more. Lawmakers also don't have answers about how the entire system would be enforced, particularly in compelling illegals that have been here less than five years to come forward.

"Great questions. ... No one seems to know," Sen. Sam Brownback, R-Kan., told FOX News. Brownback said that he thought a thoroughly worked-out "roots concept" could "appeal to a broader base" of Republicans.

Sen. John Cornyn's spokesman Don Stewart said his boss is skeptical of the suggested compromise.

"It's a matter of giving amnesty to 8 million people or giving amnesty to 12 million people. It's still amnesty to millions of people," Stewart said.

Cornyn, R-Texas, has offered his own immigration bill with Sen. Jon Kyl, R-Ariz. That proposal would give illegal immigrants up to five years to leave the country, before they can return legally to apply for permanent residence or be guest workers.

Senate Judiciary Committee Chairman Arlen Specter, R-Pa., said the "roots" compromise involves something that was discussed in his committee last Monday when the bill was being put together. He added that the "point of entry" question is also currently the subject of debate on the Senate floor. Some lawmakers prefer making people return to their home country in order to apply for legal residence.

In a separate news conference on Tuesday, Kennedy, Specter and Democratic Sens. Joseph Lieberman of Connecticut and Ken Salazar of Colorado said the McCain-Kennedy bill, besides offering a guest worker plan, is tough on border security, despite what critics say.

Critics of the bill charge that it's soft on border enforcement, but Specter and Kennedy noted that it's nearly the same as the House bill, although it omits the provision that makes felons of the illegal immigrants currently in the United States.

Kennedy said his bill changes the focus of the Border Patrol to smuggling rather than just chasing down border jumpers.

"We think it's a waste of resource to have highly skilled, highly trained border guards chasing after gardeners as they are today across the sandy desert, climbing fences," Kennedy said.

Kennedy acknowledged that his bill does not have the necessary votes to overcome a filibuster, but predicted it would eventually pass.

"Momentum is moving in our direction," said Kennedy.

"I want to get this bill finished this week ... and if we can agree among ourselves on the Republican side, we'll talk to the Democrats, but we're working very hard on it," added Specter.

Despite the calls for a final deal to be completed this week, Sen. Jeff Sessions, R-Ala., predicted no bill would be voted on by week's end. Martinez said one member floated the idea of putting this whole debate off until after May, but that appeared to land with a resounding thud.

"No one really wants to wait, but we also don't want anything rammed down members' throats," Cornyn said.

FOX News' Trish Turner and Major Garrett and The Associated Press contributed to this report.