Two Senate rebels jumped into Congress' cut-the-deficit competition on Tuesday, proposing to raise the age of Medicare eligibility to 67 and increase monthly premiums for millions of current beneficiaries.

"We can't save Medicare as we know it," said Sen. Joseph Lieberman, I-Conn., who authored the plan with Republican Sen. Tom Coburn of Oklahoma. "We can only save Medicare if we change it," he added in an apparent jab at President Barack Obama and congressional Democrats.

Democrats reacted with criticism of the proposal, which Coburn said was designed to rescue the financially imperiled program and help the nation confront a "wall of debt." Republicans betrayed no sign of support either.

If nothing else, the response underscored the difficulty of legislative free-lancing at a time the Obama administration and congressional leaders are struggling to negotiate a compromise that cuts future deficits and clears the way for an increase in the nation's $14.3 trillion debt.

Without a debt limit increase by Aug. 2, Treasury Secretary Timothy Geithner has warned, the government could default, risking calamity for the U.S. economy and serious effects worldwide.

Republicans walked out of bipartisan talks last week but nevertheless said negotiations had been fruitful. In the days since, Obama has stepped up his personal involvement in the effort.