The effort to cobble together a bipartisan immigration bill has forged unlikely alliances among some of the most influential members of the Senate, including liberal Democratic Sen. Edward Kennedy of Massachusetts and the chairman of the Republican Party, Sen. Mel Martinez of Florida.
With the exception of Sen. John McCain, R-Ariz., who has been under fire within his party for his part in the bill, the Senate's six declared presidential candidates have not been among them.
As a bipartisan team of negotiators worked Thursday on a deal to resume the immigration debate later this month, the lesser roles played by the Senate's most ambitious members illustrates the hurdles the legislation faces.
All six of the Senate's declared presidential candidates voted for last year's bill. This year, though none has ruled out voting for the bill, several have offered amendments that sponsors said would unravel the compromise.
"It would be nice to see more leadership," said Tom Snyder, who directs the political action committee for UNITE HERE, one of the unions pressing for passage of the immigration bill.
Stephen Hess, a political science professor at George Washington University, said the battle to win their parties' presidential nominations is forcing the senators away from the political center.
"They start to think: how will this appear to the basic constituency of my party? How will this be played by my opponents?" Hess said. "They become exceptionally cautious."
The changed atmosphere was dramatized during a testy late-night floor exchange last week — one day before Senate Majority Leader Harry Reid, D-Nev., pulled the bill from the Senate agenda.
The antagonists were Sens. Barack Obama, D-Ill., and Lindsey Graham, R-S.C. In the last Congress, the two were co-sponsors of a bipartisan immigration bill.
As Obama offered an amendment that would have put a five-year expiration date on a key provision of this year's bill, Graham accused the Democratic presidential candidate of abandoning "everybody over here who has walked the plank" by defying political supporters on the right and left. Graham had kind words for Kennedy's leadership but told Obama: "You are going to destroy this deal."
Obama insisted he wasn't. Yet, when the roll was called on his amendment, all of the architects of the proposed immigration deal voted against it, including Kennedy. Obama's amendment failed on a 55-42 vote.
Sen. Hillary Rodham Clinton, D-N.Y., played no part in the bipartisan, behind-the-scenes negotiations on the immigration bill and waited until 10 days into the debate to speak on it, even though she represents a state with the second-highest percentage of foreign-born residents in the nation.
Clinton spokesman Philippe Reines said the senator has been "very active in terms of championing her amendment, which is the top priority for the immigrant community."
The amendment, which would have eliminated caps on immigration for the spouses and children of legal permanent residents, was rebuffed as a deal breaker by the bill's sponsors. "This compromise was constructed very carefully and very painfully," Sen. Arlen Specter, R-Pa., told her.
Last year Sen. Sam Brownback, R-Kan., was one of the architects of the bipartisan immigration bill. He offered a passionate defense of the provisions giving illegal immigrants a chance at citizenship.