WASHINGTON—Congress's last-gasp deficit-cut talks broke into two tracks Friday, with members of a special committee continuing to seek an elusive $1.2 trillion deal while party leaders discussed a smaller, backup plan in case they fail.

Talk of a Plan B reflected the bleak outlook facing the committee as it heads into its final days. It technically has until Wednesday, but the real deadline is midnight Monday, since members by law need 48 hours to scrutinize any plan before voting on it.

If Congress falls back to a smaller plan, or stumbles completely, automatic spending cuts would kick in to bring the total deficit cut over 10 years to $1.2 trillion. Such an outcome would be seen as a failure, including by lawmakers who created the committee with the express purpose of hammering out a sweeping deficit-cutting deal that has eluded them all year long.

Committee members plan to meet through the weekend. On Friday, some were holed up for hours in the rooms of the Senate Foreign Relations Committee, which is led by Sen. John Kerry (D., Mass.), a supercommittee member.

A growing disappointment gripped the Capitol as reality set in that the hopes of a deal were fading fast, leaving lawmakers likely to return to their districts for the Thanksgiving holiday facing unhappy voters.