WASHINGTON (Reuters) - Key lawmakers have reached a framework plan to identify pension plans at risk of default as part of new funding rules in a broad pension reform bill, a Republican senator in charge of the negotiations said Tuesday.

"I think there's a common framework," Sen. Mike Enzi of Wyoming told reporters. He declined to give details.

House and Senate negotiators are working behind closed doors to finalize a bill that would put the traditional pension system on a sounder footing. Defined benefit plans, which pay a fixed amount to retirees, are underfunded by $450 billion.

Sen. Max Baucus, a Montana Democrat participating in the talks, said there was a tentative agreement on the definition of when pension plans are at risk, or in such bad financial shape that they must add cash.

The issue has bedeviled Senate and House lawmakers since they began working on a compromise version of the pension legislation in March.

Enzi and Baucus spoke after a negotiating session that also involved Massachusetts Sen. Edward Kennedy.

But Baucus said there were still questions to be answered about the framework plan. He also declined to give details but indicated the consensus was along the lines described by House and Senate aides.

Aides have said lawmakers would define a company's pension plan as at risk of default if it were less than 80 percent funded. The funding threshold could be lowered to 70 percent if the calculation were done using a stricter formula.

Employers and labor groups said Monday they were worried that proposals lawmakers were examining would be too harsh and would encourage companies to abandon offering pensions altogether.

One goal is to identify the lagging companies in traditional "defined-benefit" pension plans so as to make them better fund their plans. Some 44 million Americans rely on the system.

Enzi hoped to have the compromise bill finished before a one-week congressional recess at the end of this month.

But more details must be worked out.

Struggling airlines such as Northwest Airlines (down $0.01 to $0.58, Charts). and Delta Air Lines (unchanged at $0.75, Charts) want more time to make payments into their plans.

Another issue is whether to provide legal protection for "cash balance" pensions, which gained notoriety in 2003 when a judge ruled IBM's (up $0.47 to $78.14, Charts) plan was age discriminatory.

The agency that insures defined benefit pension plans, the Pension Benefit Guaranty Corp., is running a $22.8 billion deficit.

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