The Senate will likely begin consideration this week of a bipartisan fallback plan to avert an unprecedented U.S. default of its debt, senior Democratic aides said on Sunday.

The measure is based on a proposal first offered last week by Senate Republican leader Mitch McConnell. McConnell has been negotiating with Senate Majority Leader Harry Reid on ways to make the proposal more palatable to Democrats.

Aides said supporters expect the Democratic-led Senate to pass the legislation, but it's unclear if the Republican-led House of Representatives would provide its needed concurrence.

Right now, things are fluid. The plan isn't yet done, a senior Democratic aide said.

As initially proposed by McConnell, the measure would authorize Obama to raise the debt limit by $2.5 trillion -- without any mandatory spending cuts -- provided Obama's fellow Democrats go along with it.

Such a move would spare Republicans from having to back an increase in the U.S. borrowing authority, an unpopular move, yet protect them from being blamed for a default.

Reid wants up to $1.5 trillion in mandatory spending cuts, along the lines of those identified by a deficit-reduction group headed by Vice President Joe Biden. Staff to McConnell and Reid have also discussed creating a bipartisan committee on long-term deficit reduction, aides said.

Democratic aides said while the Senate will likely begin debate on the measure this week, it is unlikely to come up for a vote on passage until next week.

We don't have all the details yet. We are still all talking. Everything is fluid, an aide said.

An aide said McConnell and Reid staffers are working under the assumption that White House-led talks with Republican and Democratic leaders won't reach an agreement of their own.

Barring a breakthrough, this is going to be the plan, the aide said. We are working under the assumption that there won't be a plan.

(Reporting by Thomas Ferraro; Editing by Vicki Allen)