Republicans and Democrats will next week resume debating immigration in the Senate, but their compromise proposal will likely be pulled up short when it reaches the House.

The Senate bill, which will strengthen border security and give as many as 12 million illegal aliens living in America a chance to win citizenship, contrasts starkly with the House bill passed last December, which addresses only the law enforcement dimensions of illegal immigration.

Senators expect to reach a consensus before long. “The truth is, there’s basic agreement on the outlines of it,” a Senate staffer who wished to remain anonymous said yesterday. “Lots of money for security, and the guest worker program that allows earned citizenship.”

The greatest sticking point thus far has been the amendments to the bill. “[Republicans] want to do a bunch of amendments, [Democrats] want to do fewer,” the staffer said. “There have been several hundred proposed … The question is, do we vote on 20, or two or three?”

One amendment particularly contentious among senators would make illegal aliens convicted of felonies ineligible for citizenship, a measure Republicans generally favor and Democrats generally oppose.

Senator Frist, a Republican of Tennessee, and Senator Reid, a Democrat of Nevada, agreed yesterday to a deal under which the Senate would send 14 Republicans and 12 Democrats to conference with the House, of whom seven Republicans and five Democrats will be chosen from the Judiciary Committee.

Senator Cornyn, a Republican of Texas, is among those tapped, and other likely candidates are Senator Kennedy, a Democrat of Massachusetts; Senator Kyl, a Republican of Arizona, and Senator McCain, a Republican of Arizona. Mr. Kyl and Mr. Cornyn favor a more stringent policy toward illegal immigrants in the country, while Mr. McCain and Mr. Kennedy have taken positions that approach amnesty.

While the obstacles to the bill in the Senate appear surmountable, its prospects in the House remain bleak.

Representative Tom Tancredo, a Republican of Colorado, and many of the 92 members of his Immigration Reform Caucus favor more severe measures both toward illegal immigrants caught in America and those attempting to become legal citizens. Two of Mr. Tancredo’s allies also hold posts on the House Rules Committee, which would enable them to influence the floor debate.

Mr. Cornyn acknowledged the challenges ahead in a statement issued yesterday. “The differences between the House and Senate bills are significant, and the solutions will be difficult,” he said. “But it’s important to have border state Senators on this committee to help craft a final bill that meets our national security and economic needs.” Like Mr. Cornyn, most senators hope to resolve their differences in a manner that strikes a balance between all considerations: border security, the demand for labor, and the yearnings of those who hope to become American citizens.

By contrast, many representatives face single issue districts, where the predominance of organized labor or other constituencies makes it difficult, if not impossible, to compromise.

Nonetheless, key representatives see promise in the upcoming debates. “I am delighted to see the Senate return to consideration of the comprehensive immigration reform bill,” Representative Jim Kolbe, a Republican of Arizona, who favors greater leniency toward illegal immigrants already living in America, said. “My hope is that the House will be able to cast a vote this year on a complete immigration bill, which I believe will pass.”

Observers outside the Beltway were less optimistic. “Anything that looks like a potential amnesty would not have a chance,” an officer of a major agricultural company said. “That’s what’s been in the press, and that’s what I understand from the congressmen I’ve spoken to directly.”