WASHINGTON (AP) - The top Senate negotiators on the effort to prevent the government from going over the "fiscal cliff" offered a pessimistic assessment Sunday barely 24 hours before a deadline to avert tax hikes on virtually every worker.

Senate Republican leader Mitch McConnell said he's yet to receive a response to an offer he made on Saturday evening to Majority Leader Harry Reid, D-Nev., the top Democratic negotiator. The Kentucky Republican said he's reached out to Vice President Joe Biden in hopes of breaking the impasse and a McConnell spokesman confirmed the two have spoken.

Despite indications of progress in the negotiations, Democrats said Republicans were proposing to slow future cost of living increases for Social Security recipients as part of a compromise to avoid the cliff. Democrats rejected the idea.

The pessimistic turn came as the House and Senate returned to the Capitol for a rare Sunday session. The fate of the negotiations remained in doubt, two days before the beginning of a new year that would trigger across-the-board tax increases and spending cuts that leaders in both parties have said they want to avoid.

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