WASHINGTON (CNN) -- The Senate is expected to vote this morning on a delicately crafted immigration reform bill that survived a day of legislative wrangling on Wednesday that one lawmaker likened to "trench warfare."

One senator predicted the vote would be "a cliffhanger."

The bill provides a path to citizenship for some of the 12 million illegal immigrants in the U.S. and toughens border security.

The Senate will first vote to cut off debate and refer the bill for a final vote. The cloture vote requires a three-fifths majority, or 60 votes.

The final vote needs only a simple majority -- or 51 votes -- to pass.

"I think it's razor-thin," said Sen. Jeff Sessions, R-Alabama, an opponent to the bill. "There were 64 votes to proceed to this bill. A number of people I'm confident voted to proceed to it to allow the debate to go forward. But I don't know that they will be voting for cloture."

Supporters beat back a number of potentially fatal amendments on Wednesday and will face more of the 27 amendments senators agreed to before starting debate.

Proponents won a major victory with defeat of an amendment removing the bill's most controversial feature -- a path to legalization and eventual citizenship for illegal immigrants already in the country, which critics charge amounts to amnesty. (Watch challenges rise and fall )

"I think most people will recognize that citizenship is the most precious gift America can provide," said Sen. Kit Bond, R-Missouri, the sponsor of the amendment. "There are many of us who believe it should not serve as a reward to those who broke the law."

Senators voted 56-41 to table his amendment, effectively killing it. However, in a sharp illustration of the political heartburn the "amnesty" debate is causing Republicans, Bond's proposal was supported by 33 of the Senate's 49 GOP members, along with eight Democrats.

Also defeated Wednesday was an amendment by Sen. Kay Bailey Hutchison, R-Texas, that would have required adult illegal immigrants to return to their home country within two years in order to apply for a new type of visa that will allow them to stay in the United States indefinitely.

Hutchison said the change would "send the major message ... that you cannot come to our country and stay illegally and eventually get regularized without ever having to apply -- according to the law -- from your home country."

But opponents of the amendment said the so-called "touchback" requirement would render the program largely useless.

"What immigrant is going to show up and register for a program if he has to take his chances on leaving the country and coming back in before he gets some kind of immigration status?" said Sen. Dianne Feinstein, D-California. "What immigrant is going to report to deport?"

In the end, the Hutchison amendment was tabled on a 53-45 vote.

Senators also turned back two Democratic amendments, from Sens. Chris Dodd of Connecticut and Robert Menendez of New Jersey, that would have made it easier for immigrants to bring family members from their home countries to the United States.

Liberal critics of the immigration bill have complained about a new points-based system that would sharply reduce the role family ties now play in decisions about who can come into the country.

By a 79-18 vote, senators shot down an amendment by Sen. Jim Webb, D-Virginia, that would have limited the legalization process to illegal immigrants who have been in the country at least four years, rather than covering all of them in the country at the end of 2006.

Wednesday's wrangling on the Senate floor was conducted under seldom-used rules designed to keep opponents of the immigration reform bill from using the legislative process to block it.

All of the changes were being handled as one overall amendment, with separate votes on each proposal, allowing leaders to keep critics of the bill from offering their own amendments from the floor.

Republican opponents have strongly objected to the procedure, even though it was agreed to by Senate Minority Leader Mitch McConnell, R-Kentucky.

"[We're] frustrated about our ability to exercise our rights as duly elected officials," said Sen. David Vitter, R-Louisiana.

The Senate could vote as soon as Thursday on a procedural motion to cut off debate and move to a final vote, which requires 60 votes to pass.