WASHINGTON – The Senate’s revived legislation to legalize millions of unlawful immigrants faces a critical test Thursday after surviving potentially fatal challenges.

Attempts from the right and left to alter key elements of the delicate bipartisan compromise failed Wednesday, including a Republican proposal to deny illegal immigrants a path to citizenship and Democratic bids to reunite legal immigrants with family members.

The Senate killed, by a 56-41 vote, an amendment by Sen. Christopher Dodd, D-Conn., to provide more green cards for parents of U.S. citizens. By a 55-40 margin, it tabled a proposal by Sen. Robert Menendez, D-N.J., to give family members of citizens and legal permanent residents more credit toward green cards in a new merit-based points system.

A make-or-break procedural vote was set for Thursday, however, as the Senate plowed through a half-dozen amendments that supporters hoped would address waverers’ concerns.

Facing determined opposition from conservatives who call the bill amnesty, leaders need 60 votes to keep the measure alive and complete it as early as Friday.

The Senate on Wednesday killed several proposals designed to answer conservatives’ concerns that the bill, championed by President Bush, is overly lenient toward illegal immigrants. Among the amendments was one by Sen. Kay Bailey Hutchison, R-Texas, to require all adult illegal immigrants to return home temporarily to qualify for permanent lawful status. The current bill requires only heads of household seeking permanent legal residency to return home to apply for green cards.

“I don’t see how I could support this bill in any form,” Hutchison said after the vote. She had characterized her proposal as a way of removing “the amnesty tag” from the legislation.

An amendment by Sen. Jim Webb, D-Va., also defeated, would have restricted legal status applications to those who have been in the United States for four years. The bill would allow anyone in the U.S. as of Jan. 1, 2007, to be eligible.

A bid by Sen. Kit Bond, R-Mo., to deny green cards to unlawful immigrants also failed.

The bill, which toughens border security and institutes a new system for weeding out illegal immigrants from workplaces, could face more challenges, even if it survives Thursday’s test.

Particularly worrisome to backers of the bill is an amendment by Sens. Charles Grassley, R-Iowa, Barack Obama, D-Ill., and Max Baucus, D-Mont., to overhaul the employee verification system.

Grasping for more GOP support, Republican framers of the bill were proposing their own, less burdensome return-home requirement for illegal immigrants. It would apply only to heads of household and would give them three years.