How long does “full and careful consideration” of an immigration bill take?

That’s a question Senate Judiciary Chairman Patrick J. Leahy, as well as the bipartisan group of eight senators working on legislation, will have to weigh as they move forward in the coming weeks with a comprehensive immigration overhaul.

Though it was easy for the Vermont Democrat to dismiss calls for a lengthy debate from longtime opponents of an immigration rewrite, such as Republican Sen. Jeff Sessions of Alabama, the decision this weekend by a key member of the “gang of eight” — Sen. Marco Rubio, R-Fla. — to demand “full and careful consideration” of any immigration bill significantly complicates Democratic leaders’ calculations.

Leahy and Senate Majority Leader Harry Reid, D-Nev., don’t want to be seen as jamming a bill through the chamber, but they also don’t want so much time to elapse that the measure loses momentum and is brought down by opponents’ ability to vilify it.

“I think there is definitely a need for a sweet spot in terms of timing,” one Senate Democratic aide said.

Even as some members of the Senate working group celebrated a crucial deal between business and labor on March 30, Rubio released a letter that he wrote to Leahy, urging the chairman to hold a fresh round of hearings so the chamber does not feel rushed into voting on complex legislation that it has not thoughtfully considered.

Rubio is the only member of the bipartisan group of eight senators drafting the bill to raise the issue with Leahy. The other members of the group are: Democratic Sens. Charles E. Schumer of New York, Richard J. Durbin of Illinois, Robert Menendez of New Jersey and Michael Bennet of Colorado and Republican Sens. John McCain of Arizona, Jeff Flake of Arizona and Lindsey Graham of South Carolina.

Rubio’s letter was sparked by a deal between the U.S. Chamber of Commerce and the AFL-CIO on a guest worker program. Previous efforts to change immigration policy have failed in large part because of disagreements over guest workers. While the guest worker agreement cleared a major hurdle for bipartisan negotiators, a final agreement on the bill isn’t likely to be announced until next week, at the earliest.

“We want public hearings, a committee markup and an amendment process on the floor,” Rubio spokesman Alex Conant told Roll Call’s Goppers blog Monday. “We need to get buy-in from [everyone.] We want people to understand what’s in the bill and what’s not in the bill.”

Aides said it would be a mistake to interpret Rubio’s letter as an attempt to sabotage momentum or distance himself from the group and its work.

Rubio views a lengthy, traditional process that includes hearings, a committee markup and an open floor debate during which senators can offer amendments as key to his ability to build and maintain conservative support for a comprehensive immigration rewrite. Indeed, the tea party favorite is viewed as one of the few people who can persuade skeptical conservatives to either support the measure or at least hold their fire.

Rubio does not have a specific timetable in mind. But anything viewed as “rushed” would violate promises he made to grass-roots conservatives and could cost his support, even if he is OK with the bill in principle.

A Democratic aide said there is no division in the group and that all the senators want an open process.

“Nothing [Rubio] said contradicts anyone’s thinking in the group,” the aide said.

However, it’s unclear exactly how many hearings will be needed to declare the process open and transparent.

Sen. Jeff Sessions of Alabama and five other Republican senators said in a March letter to Leahy that the process should take months.

“Never again can Congress pass a far- reaching proposal only for the American people to find out what’s in it later,” Sessions reiterated March 30.

But pressure is mounting for quick action on the bill. About 40 pro-immigration activists led a protest last month outside Schumer’s office over the delay in producing a bill. And a major pro-immigration overhaul rally is set for April 10 on the West Front of the Capitol.

President Barack Obama has also called on the Senate group to hurry up, but he said he expects them to produce a bill and has repeatedly applauded their bipartisan efforts.

The group, which had hoped to unveil its bill before the end of March, now says next week is the goal. The senators have been reluctant to set hard deadlines for themselves, citing the complexity of drafting legislation overhauling the nation’s immigration policy.

One Democratic aide, citing a letter to Sessions from Leahy earlier last month, noted that the Senate has held more than 40 hearings on the issue of immigration in the past four Congresses.

“We’ve held hearings,” the aide said. “One substantive hearing on the legislation and a weeklong markup is about as open a process as anyone could ask for.”

But the timeline is yet to be decided.

Leahy, in his response to Sessions, said that while he wants to take up the bill as soon as possible, he also wants to consider the bill under regular order in an open and transparent process. Like Sessions, Leahy has also been critical of the group for taking so much time to draft its bill, given the fact that it is one of the president’s biggest legislative priorities this year.

David M. Drucker contributed to this report.