House Speaker John Boehner publicly blessed a Republican deficit-reduction plan Tuesday that would raise $300 billion in additional tax revenue while overhauling the IRS code, bucking opposition by some GOP presidential hopefuls and colleagues wary of violating a longstanding point of party orthodoxy.

Boehner, the top Republican in Congress, spoke as time grew perilously short for agreement by the deficit-fighting “supercommittee.” The panel has until a week from today to vote on any compromise, but several officials said that in reality, perhaps as little as 48 or 72 hours are available to the six Republicans and six Democrats.

While Boehner’s voice is important, his endorsement does not mean that all Republicans will follow him or that a deal is in sight. Republicans have been unified for two decades in opposition to higher taxes, while Democrats on the supercommittee insist on additional revenue before they will agree to cuts in benefit programs like Medicare as part of a compromise.

The speaker said that the plan, outlined a week ago to Democrats on the committee, was “a fair offer.” Adding tax reform would generate economic growth, he said, speaking as the supercommittee groped uncertainly for a compromise to reduce red ink by $1.2 trillion or more over a decade.

Any deal must be certified by the nonpartisan Congressional Budget Office as meeting the $1.2 trillion target, circulated to lawmakers and then posted publicly before the committee takes formal action. Failure to act would trigger $1.2 trillion in automatic deficit cuts in 2013 that both sides say they want to avoid.

The full committee hasn’t met in several days, but various subgroups have been in near constant contact.

More than deficit reduction is at stake, one year into an era of divided government.

Democrats are hoping to add elements of President Barack Obama’s jobs legislation to any deficit-cutting deal, including extensions of a Social Security payroll tax cut and unemployment benefits that are due to expire at the end of the year. A comprehensive rewrite of farm programs may hang in the balance, too, and lawmakers also must pass legislation to assure sufficient funds to reimburse doctors who treat Medicare patients.

As the pace of private talks intensifies, the two sides vie publicly for the high ground in public opinion.

“I am still hopeful that a few Republicans will put their country first and come to us with a credible offer with real revenue,” Sen. Patty Murray, D-Wash., co-chair of the supercommittee, told reporters as she emerged from a late-afternoon meeting.

Earlier, the Republican Senate leader, Mitch McConnell of Kentucky, said GOP members on the committee outlined a proposal several days ago and have yet to receive a response from Democrats. “It’s been a long week waiting for a counterproposal,” he said.