The bill's authors will haggle over how stiff punishments should be for illegal immigrants.

As the Senate considers the bill, the lawmakers who wrote it will need to persuade skeptical conservatives that the plan does enough to punish immigrants who illegally entered the country.

The bill would offer a path to citizenship for most of the nation's estimated 12 million illegal immigrants.

House Speaker Nancy Pelosi, knowing Democratic backing won't be enough to pass the bill, has said it would need support from at least 70 Republicans. In the Senate, passage would require at least half of the Republicans, as well as some conservative Democrats.

Conservative critics have denounced the bill, labeling it "amnesty" -- a politically lethal charge.

When debate begins Monday, lawmakers will haggle over the penalties illegal immigrants should face to become legal residents, then citizens. Should they be forced to leave the country first? How much should they pay in fines? Should they be required to forfeit the Social Security benefits they earned while working illegally?

Sen. John Cornyn (R-Texas), who was heavily involved in the negotiations, said he worried the bill might end up being a repeat of the 1986 Immigration Reform and Control Act.

That law legalized some 2.7 million illegal immigrants, but because mandated enforcement measures were never put in place, millions more are believed to have illegally crossed the border in the hope of citizenship.

"We have to make a basic determination: Will this bill restore respect for our laws?" Cornyn said. "Or will it have the opposite effect and encourage still more disregard for our immigration and border security laws?"

Senators who helped negotiate the bill are aware of those concerns. "What more hurdles can be placed to be sure we do the maximum to avoid the charge of amnesty?" asked Sen. Arlen Specter (R-Pa.). "We are still open to suggestions."

In crafting the compromise, Specter, his bipartisan group of colleagues and two Cabinet officials had sought to address such an "amnesty" charge. Although the bill would grant probationary legal status to illegal immigrants who were in the country before Jan. 1, 2007, it requires a number of steps to become legal.