A bipartisan Senate group on immigration legislation is attempting to craft an agreement so secure that the eight members will oppose amendments to its core provisions, an arrangement that could delay the introduction of a bill, people familiar with the negotiations said.

The senators had said they hoped to present their proposals this week, but Republican members expressed skepticism about that timetable Sunday. The group continues to negotiate issues related to new visa programs for agricultural and high-tech workers and has not reached agreement on a guest-worker program for low-skilled foreign laborers, said the individuals, who spoke on the condition of anonymity to discuss private deliberations.

Sen. Lindsey O. Graham (R-S.C.) said on NBC’s “Meet the Press” that he hopes a deal can be reached “in the next couple of weeks.” A GOP Senate aide involved in the negotiations said a deal is unlikely this week because writing the bill, which is expected to be up to 1,500 pages, is going slowly.

The timetable is important because President Obama has called for a comprehensive immigration reform measure that could be approved by the Senate and House in time for him to sign it into law by the end of the year. Under the timetable eyed by the Senate group, whose proposal is expected to be the template for a potential deal, the legislation would be introduced in the Judiciary Committee this month for the amendment process.

The goal is to get the measure to the Senate floor after Memorial Day. Sen. Charles E. Schumer (N.Y.), the lead Democrat in the bipartisan working group, remained optimistic about the time frame.

Sen. Lindsey O. Graham (R-S.C.) said on NBC’s “Meet the Press” that he hopes a deal can be reached “in the next couple of weeks.” (Alex Wong/Getty Images)

“Thus far, we are on track,” he said Sunday on CBS’s “Face the Nation.” “All of us have said that there will be no agreement until the eight of us agree to a big, specific bill. But hopefully we can get that done by the end of the week.”

Obama has supported the Senate group’s effort, but the president has vowed to step forward with his own legislative proposal if the senators are unable to deliver a plan in the spring.

The senators are mindful of the pitfalls of previous immigration reform efforts, when amendments by supporters and opponents in 2007 helped doom a bill in the Senate. In an attempt to head off such threats, the Senate group is trying to strike a deal in which all the members agree to oppose any amendments to the core provisions.even if they might agree with the amendments, people familiar with the talks said.

The group is concerned that if one provision is amended, the entire bill will fall apart because the deal is predicated on a comprehensive plan composed of carefully negotiated pieces.

The core provisions are likely to include a path to citizenship for illegal immigrants, new visa programs for low- and high-skilled workers and for agricultural workers, a reduction in some family visa categories, and increased border control and workplace security.

But ironing out the specific language at a time that the senators are still negotiating portions of it has made the drafting process laborious, said one member of the business community who has talked with the senators.

“My sense is this week is pretty unlikely,” this person said. “Just the mere writing is enough to delay past this week, but that’s not the problem. There are still substantial disagreements on almost all the major parts.”

Another labor community individual was more optimistic about the progress, but agreed that the bill would probably not be ready this week. This individual said that the senators are trying to determine which areas of the bill would be considered “core provisions” on which they will agree not to support amendments.

“No one wants there to be errors,” the individual said. “It’s a contentious process.”

Immigration advocates have begun to grow impatient, and thousands are expected to attend a rally Wednesday at the Capitol. Republicans, however, have cautioned against excessive haste.

Sen. Marco Rubio (R-Fla.), a member of the group who has emphasized that a deal has not been reached, received permission from the group to brief the GOP Senate caucus on the progress of the negotiations, something he probably will do Tuesday, aides said.

“He’s talking to a lot of people,” the business community individual said. “Partially, it’s over how to resolve some of these issues, but it’s also to find out how toxic this thing could be.”

Discuss this topic and other political issues in the politics discussion forums.