The Senate will spend this week reshaping an immigration-reform bill in hopes of building bipartisan support for the measure. But that detailed scrutiny of the measure is scaring off the very lawmakers that its authors hoped to win over.

By the time the Senate Judiciary Committee votes on the bill later this month, dozens of amendments will have been added to address the concerns of lawmakers now reluctant to back it. But it's still not clear whether the changes will satisfy reluctant lawmakers without scaring off those who support the original measure.

"Even though there has been some support during the amendment process, it's unclear on whether the immigration bill will make it over the finish line," Ron Bonjean, a GOP consultant and former top Senate and House aide.

Democrats hold 55 Senate seats, five votes short of the 60 needed to advance the reforms, but they will need even deeper Republican support, as many as a dozen votes, because a handful of Democrats facing re-election in 2014 -- including Sens. Mark Pryor, of Arkansas, and Kay Hagan, of North Carolina -- may not heed President Obama's call to revamp the immigration system.

"The Democrats from red states where immigration reform is unpopular won't vote for it," said Jennifer Duffy, senior editor for the Cook Political Report.

There is already some Republican support for immigration reform in the Senate, including Sen. Marco Rubio, R-Fla., and other members of the so-called Gang of Eight, which worked out a bipartisan compromise that they are now trying to convince colleagues to support.

Many conservatives have resisted reform efforts because they see citizenship for illegal immigrants as amnesty for lawbreakers. Senators still making their way through the 844-page bill also say it doesn't do enough to secure the U.S.-Mexico border. Fanning the flames of opposition, the conservative Heritage Foundation last week released a report showing that immigration reform would cost the nation $6.3 trillion.

Making it even harder to win over Republicans, a Senate committee last week dropped the one provision that allowed some Republicans to support immigration reform, a central provision that required the border be secured before other provisions, including citizenship, can take effect.

Sen. Jeff Sessions, R-Ala., a critic, compared the bill to a mackerel left out in the sunshine. "The longer it's out there, the worse it smells," he said.

A Democratic amendment from Judiciary Committee Chairman Patrick Leahy, of Vermont, that would allow gay U.S. citizens to bring foreign partners to the U.S. and grant them green cards, may also cost Senate leaders additional Republican support.

Republicans who do support immigration reform are embracing a provision in the measure that would grant more visas for high-tech workers and provide a mechanism that would make it easier for U.S. farmers to bring in immigrant farm laborers.

In addition to their Democratic colleagues, Republican lawmakers are also under pressure from business groups, including the U.S. Chamber of Commerce, that say American businesses need greater access to immigrant labor.

Microsoft Corp., for example, spent $8 million in 2012 lobbying for an immigration-reform bill because it needs more high-tech workers.

Other business groups are also spending big money to pass the measure, dwarfing the immigration opposition lobby.

"This is their year and they are throwing everything at it," said Roy Beck, founder and CEO of NumbersUSA, which seeks to limit immigration.