Months of tumultuous negotiations with the White House and GOP allies have brought the Senate's liberal lion, Edward M. Kennedy, to the brink of passing a bill to legalize up to 12 million unlawful immigrants.

But his concessions to get there have alienated liberals who in the past have counted him as their strongest champion. A showdown test vote is scheduled Tuesday, and the Senate could pass _ or reject _ the bill by week's end.

Traditional Kennedy allies are mystified and angry at the Massachusetts senator's willingness to accept Republican-backed measures such as subjecting illegal immigrants to steep fines and trips home, separating immigrants from relatives and letting new guest workers stay only for short periods of time with little chance of citizenship.

"I think that in his heart, he's where I'm at, but he wants to see a deal move forward and he's willing to take certain steps that I might not be willing to take," said Sen. Robert Menendez, D-N.J., who abandoned the deal just before it was announced because it scrapped many immigrants' ability to come to the U.S. based solely on family ties.

"In the pursuit of moving us along, he's probably swallowed hard on some things that he himself would not have accepted" otherwise, Menendez added.

It's a familiar spot for Kennedy, 75, whose standing as a liberal firebrand during his 45 years in the Senate belies his history of partnering with Republicans on major domestic agenda items.

He's done so twice before with President Bush, on the No Child Left Behind education law and a broad Medicare prescription drug overhaul. In both cases, Kennedy was accused by liberals of compromising too much in the interests of a deal.

"You can hold to rigid positions in the United States Senate _ and I respect that _ and get nothing done, or you can try and find common ground," Kennedy said in an interview with The Associated Press.

He regards an immigration overhaul as the civil rights imperative of the 21st century, and sees the same legacies of prejudice and discrimination standing in the way. Legalizing 12 million unlawful immigrants "is worth the fight," Kennedy said.

Kennedy's pragmatic history and his expertise _ he maneuvered a broad immigration overhaul through the Senate in 1965, during his second term _ has earned him Bush's trust.

"Senator Kennedy is one of the best legislative senators there is. He can get the job done. I know firsthand, because we reformed our education system," Bush said at a March news conference in Mexico.

Some activists who revere Kennedy privately voice a sense of betrayal at the lengths to which he has been willing to go in search of a deal.

The AFL-CIO condemned the bill last week, and its leaders have harsh words for the senator they trusted to shepherd a historic immigration measure.

"I am angry," said Ed Sullivan, president of the labor federation's building and construction trades department, and a Massachusetts native who describes Kennedy as a "good friend." "We can't understand how our senators would support this."

Sullivan said Kennedy's intentions were good, but his pragmatism drew him into a bad deal.

"I think he's locked in. He's a legislator that likes to pass legislation," Sullivan said.

In early March, representatives of liberal groups angrily cautioned Kennedy against starting negotiations with Bush's team and a group of senators led by conservative Jon Kyl, R-Ariz., on an immigration compromise that could attract GOP support. Kennedy contended it was the only way to craft a bill that would survive.

Months later, just before Kennedy went before news cameras to announce his breakthrough immigration deal with Republicans and the White House, some of them complained he had agreed to a shabby bargain that would rip families apart and sentence millions more immigrants to exploitation from abusive employers.

"We were saying, 'Senator, we think you're going too far,'" said Frank Sharry, executive director of the National Immigration Forum. "He told us, 'You're not going far enough. If you want to get this done, you've got to get real. I've been around this place 40 years. This is the best we can do. If you want to get it done, follow me.'"

He may have sounded unshakable, but Kennedy admits to some moments of doubt during the roller coaster process of crafting the bill.

"Of course there are times when we really wonder whether it continues to make sense to engage in this," Kennedy said. "It's a battle."

Ultimately, though, he said, "I believe in the legislation. The alternative to this is nothing, and that's, I think, completely unacceptable."

Kennedy's involvement has made the immigration deal more difficult for some Republicans to stomach. Conservative critics of the plan brand it the "Kennedy-Bush amnesty" program, and invoke the Massachusetts senator's name to paint it as a far-left solution.

Sen. Trent Lott, R-Miss., has weathered bitter criticism back home for supporting the measure. NumbersUSA, an anti-immigration group, ran ads last week that juxtaposed Lott's picture next to Kennedy's and said Lott had "joined with Ted Kennedy in strong-arming senators to support amnesty for millions of illegals, many of whom have already taken jobs from Mississippi workers."

Kyl called working with Kennedy "a real education."

"He's a tough bargainer. He's a real strong advocate for some points of view with which I disagree, but I have found him to be a man of his word, and on the emotional and difficult issue of immigration reform, that's a big deal," Kyl said.

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The bill is S 1639.