With bipartisan negotiators slowly inching toward a possible deal on comprehensive immigration reform, Senate Majority Leader Harry Reid (D-Nev.) on Monday filed for cloture on last year’s Senate-passed reform package, setting a hard deadline of Wednesday morning for a deal to be reached.

Although lawmakers and aides involved in the talks said negotiators are still struggling to work out an agreement on several key issues, one GOP aide said the tide appears to have shifted slightly in favor of a deal.

“I think the feeling changed over the weekend from likely no to likely yes,” the aide said. However, the aide expressed concern that Reid’s deadline could mean lawmakers end up being forced to agree to take up the issue based on descriptions of any final deal without actually ever seeing the legislation.

“Whether it’s a good deal or [if conservatives] could support it [is] still in the air because we have not seen any actual language yet,” the aide cautioned, adding “the problem is the timing — it wouldn’t be a problem if there wasn’t this self-imposed deadline from Reid.”

Talks continued throughout last week at the Member level — including a late-Friday session reportedly involving Sens. Arlen Specter (R-Pa.) and Edward Kennedy (D-Mass.) as well as Bush administration officials — and aides met again Monday morning.

But with one day left before Reid’s Wednesday deadline, Kennedy and others involved in the talks said it was still unclear whether a deal can be done.

Kennedy, who declined to comment specifically on the session with Specter, did say Monday that all involved have continued to work earnestly toward a deal and that Members are committed to working up until the deadline. But Kennedy acknowledged that if a deal cannot be cut before then, it may not ever get done. “We have to get it done by then,” Kennedy said.

With talks likely to continue up until Wednesday morning, one key issue Republicans will have to decide today is whether they will vote for cloture on last year’s bill, which Reid has said could be used as a vehicle for a new compromise. Republicans have opposed Reid’s decision to use that bill, and as late as Friday they seemed unanimously opposed to supporting cloture unless a formal deal had been reached.

But with negotiators making some progress over the weekend, that dynamic could change and Republicans could be inclined to back cloture even without seeing the deal. A “big topic at tomorrow’s lunch will be whether or not to vote for cloture without seeing what this ‘compromise’ actually looks like — and yet giving up our biggest negotiating tool,” a GOP aide said Monday.