WASHINGTON (CNN) — The U.S. Senate approved an immigration bill Thursday that would toughen security at the Mexican border and grant many illegal immigrants a path toward citizenship.

The action set the stage for another battle as lawmakers try to mesh it with the stricter bill passed by the House in December.

The House version solely focused on border security and enforcement immigration law and did not contain any legalization provisions, which many conservatives, especially in the lower chamber, decry as “amnesty.”

The House bill also contains a controversial provision that would make illegal immigration a felony.

The Senate legislation has a temporary-worker program, which President Bush supports but is not part of the bill the House approved.

The Senate voted Wednesday to cut off debate on its measure, setting the stage for Thursday’s vote.

Senate Majority Leader Bill Frist, R-Tennessee, said that “the most contentious part” of the bill is handling the millions of immigrants now in the country.

Under the Senate bill, those who have been here two to five years would enter a temporary-worker program, while those here longer would be eligible for legal status or citizenship after an 11-year probationary period.

“I do support that provision as written in the bill,” Frist said about allowing illegal immigrants a path toward citizenship.

Sen. John McCain, R-Arizona, said, “I think it’s really the only viable solution that you can arrive at, given that there’s 11 million or 12 million people who are already here illegally.

“You can’t send them back. The status quo is not acceptable, so you have to find a way to, I think, not forgive them and not say, ‘Look, you can just be citizens,’ but give them a very tough path to citizenship,” McCain told ABC’s “Good Morning America.” (Watch McCain explain why he’s proud of the immigration bill — 4:55)

Under the Senate legislation, Illegal immigrants in the United States less than two years would be sent to their home countries.

Sen. Sam Brownback, R-Kansas, said that while he sees the bill easily clearing the Senate, reconciling it with the House version will require tough negotiation and compromise.

“I think one of the requirements is that it has to be a comprehensive approach, that you’ve got to deal with the 11 million that are here illegally, you have to deal with the future flow, and you have to secure the border,” Brownback said Thursday on “Fox & Friends.”

“If we do one, the rest of it isn’t going to work, and you’re going to be back in the same soup in a few years,” he said.

McCain said he was optimistic the plans can be reconciled.

“We’re not drawing any lines in the sand. And I think that already we are hearing at least some proposals from the House side that deserve consideration and certainly show some movement on their part,” he said on ABC.

House Majority Leader John Boehner, R-Ohio, noted the gulf between the two versions of the bill.

“We have two very separate and distinct directions that we’re going [in] when it comes to controlling our borders, enforcing our laws, where the House was, where the Senate has provisions that go far beyond that. And I don’t underestimate the difficulty in the House and Senate trying to come together in an agreement,” Boehner told a news conference Thursday.

“But I’m hopeful that we will come to a resolution and pass a bill.”

Boehner stressed the importance of the border issue to the House. “You can’t control the problem without first strengthening the borders and beginning to enforce the laws,” he said.

Frist had once supported the House version of the bill, which would send all illegal immigrants to their countries.

But the likely 2008 Republican presidential candidate said he has changed his position because “a mature understanding” over the handling of illegal immigrants has emerged in the Senate after two weeks of debate.

“Many have been assimilated into our society; 40 percent have been here longer than 10 years, have had kids go to high school and college and now have jobs,” Frist said. “And therefore, we put together a compromise.”

With Senate passage inevitable, a group of six Republicans held a news conference Thursday to stress their opposition to the bill — and to ask that the House moderate its stance in conference committee.

“This legislation, I think, is well outside of what I would consider responsible reform. It’s misfocused. It puts the cart before the horse,” said Sen. Rick Santorum of Pennsylvania, speaking with Sens. Jon Kyl of Arizona, George Allen of Virginia, John Cornyn of Texas, Jim DeMint of South Carolina and David Vitter of Louisiana.

Santorum said any final bill must include provisions for border security, employer verification of employees’ immigration status and a temporary-worker program that fills needs in this country.

As the discussion continued in the U.S. capital, Mexican President Vicente Fox, on whose border a 370-mile fence would stand, was on a swing through three American Western states.

“Mexico believes that it will take more than just enforcement, building walls, to really solve the challenge posed by the migration phenomenon,” Fox said Wednesday in Seattle, Washington, according to The Associated Press.