WASHINGTON — It was a fitting revival for the on-again, off-again bipartisan immigration compromise.

Shortly after Senate Democrats and Republicans breathed life into the measure through a crucial vote Tuesday, it ran into more delays, postponing any real action until another day.

The bill, which would potentially legalize millions of unlawful immigrants, faces the same threats that derailed it earlier this month. The White House and Republican and Democratic architects of the bill predicted approval of the measure by week's end.

Negotiators continued haggling over key elements of the bill even after the test-vote, giving conservative GOP foes an opening to pounce. The opponents succeeded in delaying until Wednesday consideration of a package of amendments designed to pave the way for a final vote on the bill. They did so by using Senate rules to insist that the entire 373-page proposal be read aloud, relenting only when Majority Leader Harry Reid, D-Nev., agreed to postpone votes.

"It's only fair," Reid said, chalking up the delays to the slow-grinding rhythms of the Senate.

It was just the first in a series of formidable obstacles lying in the bill's path. The Senate is slated to consider 26 amendments — mostly from senators seeking to change key elements — that have the potential to either sap its support or draw new backers.

After that, the legislation must overcome another make-or-break vote as early as Thursday. And there is no guarantee that it will ultimately attract enough support to pass.

Republicans and Democrats alike are deeply conflicted over the bill, which also would create a temporary worker program, strengthen border security and institute a new system for weeding out illegal immigrants from workplaces.

Masking those divides, the Senate voted 64-35 to revive the bill, which stalled earlier this month when it failed to muster the 60 votes it needed to scale procedural hurdles.

Twenty-four Republicans joined 39 Democrats and independent Sen. Joseph I. Lieberman of Connecticut to move ahead with the bill. Opposing the move were 25 Republicans, nine Democrats and independent Sen. Bernard Sanders of Vermont.

Sen. Edward M. Kennedy of Massachusetts, the lead Democratic negotiator on the bill, called the vote "a major step forward for our national security, for our economy and for our humanity."

"We did the right thing today because we know the American people sent us here to act on our most urgent problems. We know they will not stand for small political factions getting in the way," Kennedy said.