Updated

The top Republican on the Senate Judiciary Committee called on his colleagues Thursday to block a controversial immigration bill over concerns about the process Democrats were using to advance it.

Senate Democratic Leader Harry Reid has pushed four different versions of the immigration bill known as the DREAM Act without a hearing. The latest was introduced on Tuesday, with Democrats teeing up for a test vote potentially within a week.

But Sen. Jeff Sessions, R-Ala., called the process "reckless" and urged the Senate to oppose cloture -- a procedural hurdle that requires 60 votes.

"Not one of these bills has been reviewed by the Judiciary Committee, nor have senators been provided with a CBO score. This shell game makes it nearly impossible for members of this body, and their constituents, to properly review and consider the legislation prior to a vote," Sessions wrote in a letter to his colleagues. "It is an abuse of the process and on that basis alone members ought to oppose cloture."

Sessions opposes the bill anyway and has decried it as an "amnesty" measure. The bill would give young illegal immigrants who attend college or join the military a pathway to legal status, but Sessions argues that the eligibility standards are far too broad.

The four versions of the bill that have been filed are similar and bear the same name. A Senate source said the latest version would lower from 35 to 30 the age at which an illegal immigrant would be eligible to go through the program. The new version also made illegal immigrants who had committed marriage or voter fraud ineligible, the source said, though those responsible for other infractions like document fraud could still apply.

The changes could be a sign that Democratic leaders are watering down the bill in response to behind-the-scenes grumbling in the Democratic caucus. But the changes appear unlikely to win much support from the Republican side.

All 42 Senate Republicans signed a letter Wednesday vowing to block any legislation until a government spending bill is passed and the Bush tax cuts, set to expire at the end of the year, are extended. A couple Senate Republicans could cross over once those issues are resolved, but Reid would still have defecting Democrats on his hands.

Sen. Ben Nelson, D-Neb., wrote in a column on his Senate website that he would oppose the plan to give "hundreds of thousands of undocumented immigrants" a path to residency.

Democratic groups, though, are putting on the pressure. Organizing for America sent out an e-mail urging supporters to call moderate Republican Sens. Olympia Snowe and Susan Collins, both from Maine, and urge them to vote yes.

Democratic Party Chairman Tim Kaine said the bill would help build the military, urging Republicans to support it.

"The DREAM Act is based on two very simple principles: that children brought up in this country should not be penalized for the actions of their parents and that our country is made stronger by hard-working immigrants who are willing to do what it takes to build a better life in America in a way that makes our country both stronger and more secure," Kaine said in a statement.