A breakthrough in immigration reform faces a tough road ahead with division emerging among congressional leaders over how to handle millions of illegal immigrants living in the United States.

The agreement announced Thursday by some lawmakers and the White House that would grant legal status to those illegal immigrants and increase border and interior enforcement initiatives enters its first round of debate on Monday in the Senate.

"I don't know if the immigration legislation is going to bear fruit and we're going to be able to pass it," said Senate Majority Leader Harry Reid, D-Nev., who said he had "serious concerns" about the bill.

The bill also faces challenges in the House. House Speaker Nancy Pelosi, D-Calif., said there are divisions among Democrats and she won't bring it to the House floor unless President Bush can guarantee he has 70 Republican supporters.

The plan would establish a temporary worker program for new arrivals to the United States with a separate program for agricultural workers. The bill also would include provisions for new technology to ensure against immigration document fraud.

Homeland Security Secretary Michael Chertoff said Friday that illegal immigrants living in the United States who don’t try to gain legal worker status will be forced to leave.

“The people who don't apply and don't get the Z visa are going to be hunted down and they're going to get deported,” Chertoff told FOX News. “So there's a very clear choice: You can either bring yourself into the system and find, you know, safety, pay your fine, and work within the law, or you can stay outside the law and we're going to focus our attention on those people and deport them.”

Some conservatives labeled the bill "amnesty" shortly after the announcement, saying the bill would reward illegal immigrants with a way to stay in the U.S. permanently.

"What part of illegal does the Senate not understand? Any plan that rewards illegal behavior is amnesty," said Rep. Brian Bilbray, R-Calif., chairman of the House Immigration Reform Caucus.

The bill would also establish requirements that border security and employer verification occur before illegal immigrants are legalized, which won over some lawmakers who opposed last year’s immigration reform bill because they said it was too weak.

Sen. Jon Kyl, R-Ariz., decided to support the bill because it was stronger than last year's legislation.

"So I got into the game, and fought very, very hard for what I believe, and I am confident that the provisions that I helped to get in that bill will make it a much better bill,” said Kyl.

Some liberal lawmakers don’t support the bill, saying it undercuts the ability of extended families to join immigrants who become legal workers.

“It tears at the fabric of family reunification by limiting and eliminating the ability of us citizens and lawful perm residents to petition for children, parents, and siblings to join them in this country,” said Sen. Robert Menendez, D-N.J.

Menendez and Sen. John Cornyn, R-Texas, withdrew their support of the agreement before Thursday's announcement.

The agreement, which also mandates tougher border security and workplace enforcement, marked an extraordinary marriage of liberal and conservative goals that has the potential to bridge stubborn divides and ensure enactment of new laws this year.

Sen. Edward M. Kennedy, D-Mass., who led the Democrats through negotiations, called it an example of the "politics of the possible."

The deal would allow illegal immigrants to come forward right away, but they could not get visas or begin a path to citizenship until the border security improvements and a high-tech worker identification program were in place.

After that, illegal immigrants could obtain a renewable "Z visa" that would allow them stay in the country indefinitely. After paying fees and fines totaling $5,000, they could ultimately get on track for permanent residency, which could take between eight and 13 years. Heads of households would have to return to their home countries first.

Liberals say they don't support the bill because it would admit future arrivals into the U.S. based on their skills, education levels and job experience over family ties.

"We have concerns about the historic shift away from family unification as the backbone of our immigration system," said Kevin Appleby of the U.S. Conference of Catholic Bishops.

Immigrant advocacy and labor groups also oppose the terms of a new guest worker program in which low-skilled immigrants would be forced to leave the country after temporary stints and would have limited opportunities to stay and get on a path to permanent legalization.

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