Washington, D.C. ? The plan to overhaul the nation’s immigration system survived its most serious challenges Wednesday, when the Senate defeated amendments to disqualify hundreds of thousands of illegal immigrants from legalization and to extend visas to hundreds of thousands more relatives of U.S. citizens and green-card holders.

But just after midnight, the Senate approved by a single vote an amendment to shut down the bill’s proposed guest-worker program after five years. The 49-48 vote, coming just days after the Senate cut the size of the guest-worker program in half, could upset the delicate bipartisan balance behind the deal.

Before that early-morning vote, the fragile coalition behind the “grand bargain” appeared to be holding together as the legislation nears final passage – but barely.

Still, the bill took a decidedly conservative turn Wednesday night with the adoption of amendments that would at once declare English the national language and designate English the “common language” of the United States. The Senate also blocked the bill’s newly legalized undocumented workers from receiving the earned-income tax credit, while denying legalized undocumented workers any Social Security benefits they may have earned after overstaying their visa.

The Senate will vote as early as tonight on whether to cut off debate on the bill and move to a final vote, possibly Friday night.