The immigration reform bill headed to the Senate floor lacks the 60 votes it needs to advance and will likely have to be partially rewritten to win congressional approval, proponents of the bill said.

Democratic and Republican sources working to build support for the bill agreed that garnering the necessary GOP votes would require amending the legislation to strengthen its border security components. Additionally, there are senators on both sides of the aisle who don't support the current measure because of provisions other than border security who also must be appeased.

The bill's supporters are now focusing their efforts on wooing as many as five Democrats who might oppose it and about two dozen Republicans described as "gettable" or "maybes" but who for now are far from a "yes" vote. (Senate Minority Leader Mitch McConnell, R-Ky., is not among those targeted.) The bill is scheduled to hit the floor next Monday, and Senate Democratic leaders said it may go up for a final vote before the July 4 recess.

"They probably are close to 60, but they're not yet at 60 right now," a former Republican Senate leadership aide said. "My sense is, for that to happen, there probably will need to be changes to border security."

The GOP "gettables" include three members of leadership: Senate Conference Chairman John Thune, of South Dakota; Conference Vice Chairman Sen Roy Blunt, of Missouri; and Sen. Jerry Moran, of Kansas, who chairs the Senate Republican campaign committee. The only Republicans now classified as guaranteed "yes" votes are the four GOP members of the gang of eight, including Sens. Jeff Flake and John McCain, of Arizona; Sen. Lindsey Graham, of South Carolina; and Sen. Marco Rubio of Florida.

Whether Rubio personally offers amendments to beef up the legislation's border security proposals -- changes the senator has said he supports -- or leaves that to other Republicans, is viewed as a signal of the strength of the Floridian's commitment to the bill in its current form. Rubio has been meeting with undecided Republicans to determine what they need to support the bill; his staff is having similar discussions with their counterparts.

Republicans' primary concern is the how much authority the legislation delegates to the Department of Homeland Security. They want spelled out in the bill how border security would be measured rather than leave that decision to the discretion of the White House. Republicans insist that the border be secured before other provisions, like legalization of illegal immigrants, take effect.

Rubio's support for immigration reform is perhaps the most crucial of any member of Congress, and his actions during the Senate floor debate will be closely scrutinized.

"If they don't do something on this bill to shore up the triggers and add codification language -- whether it's real or fictional in terms of having an affect on security and how you measure that, it's going to have problems passing," said a Republican strategist who supports the bill.

One Democratic strategist working on behalf of immigration reform said Democrats who don't now support the bill include Sens. Max Baucus, of Montana; Joe Donnelly, of Indiana; and Mark Pryor, of Arkansas. Undecided Democrats include Sens. Mark Begich, of Alaska, and Mary Landrieu, of Louisiana, who, along with Pryor, face re-election in 2014.

A second Democratic strategist said proponents have a wary eye on Democrats who voted against the Dream Act - a much narrower measure that would have granted citizenship to some illegal immigrants - in 2010. Those lawmakers include Sen. Kay Hagan, D-N.C., who is up for re-election next year, and Sen. Jon Tester, D-Mont.

Sen. Joe Manchin, D-W.Va., who skipped the Dream Act vote, is expected to oppose the immigration reform bill after fighting for another issue unpopular in his home state, gun control.

Republicans on the fence include Sens. Lamar Alexander and Bob Corker, of Tennessee; Kelly Ayotte, of New Hampshire; Richard Burr, of North Carolina; Dan Coats, of Indiana; Mike Crapo and Jim Risch, of Idaho; Saxby Chambliss and Johnny Isakson, of Georgia; Orrin Hatch and Mike Lee, of Utah; Dean Heller, of Nevada; John Hoeven, of North Dakota; Mike Johanns, of Nebraska; Ron Johnson, of Wisconsin; Mark Kirk of Illinois; Rand Paul of Kentucky; Rob Portman, of Ohio; and Pat Toomey, of Pennsylvania.

"I think they are close to 60," said one Republican strategist who is working to pass immigration reform. "Some security amendments are needed to pass, but I am confident they will."

Meanwhile, Senate Majority Leader Harry Reid, D-Nev., insisted during a recent interview that he has 60 votes for the bill.

Supporters of comprehensive immigration reform have speculated that the Senate bill could garner around 70 votes, creating momentum that would boost its chances in a hostile Republican House. In fact, some of the proponents have set 70 as an unofficial benchmark, believing that weaker support in the Senate, even if the bill passes, could dampen reform's prospects in the House.

But the Democratic strategist working on behalf of the bill said some Democratic lawmakers reject the notion that 60 votes or more are needed to help build momentum for the measure. These Democrats, many of them quite progressive, say setting a 60- or 70-vote benchmark empowers Republicans to force Democrats to support conservative amendments Democrats oppose.

"There's a backlash brewing over [70 votes], because to get that [Democrats] will have to make deals on border security that many find unpalatable," the Democratic strategist said.