As the so-called super-committee continues to sputter, the problem is a familiar one: to achieve a debt-reduction deal, Democrats are willing to make concessions on entitlement “reforms,” if Republicans are willing to make concessions on tax increases on the wealthy. GOP officials, meanwhile, aren’t willing to make any concessions, no matter what Dems offer.

There are two sides to the balance sheet: revenue (money coming into the treasury from taxes) and expenditures (money the government spends). Democrats want a “balanced” approach between the two; Republicans insist the revenue side of the balance sheet be ignored altogether.

Well, most Republicans, anyway. There appear to be some slight cracks emerging in the anti-tax wall.

A group of 40 House Republicans for the first time Wednesday encouraged Congress’s deficit reduction committee to explore new revenue as part of a broad deal that would make a major dent in the nation’s debt, joining 60 Democrats in a rare bipartisan effort to urge the “supercommittee” to reach a big deal that could also include entitlement cuts.

The letter they sent represents a rare cross-party effort for the rancorous House, and its organizers said they hoped it would help nudge the 12-member panel to reach a deal that would far exceed the committee’s $1.5 trillion mandate.

Among those who signed were several dozen Republicans who had previously signed a pledge promising they would not support a net tax increase. Among the Democratic signers were some of the House’s most liberal members who have opposed entitlement cuts.

This group of 100 members didn’t go into too many details, other than to say $4 trillion in debt reduction is a worthwhile goal, and that “all options for mandatory and discretionary spending and revenues must be on the table.”

It’s worth emphasizing that the Republican signatories did not explicitly endorse any tax increases on anyone, only going so far as to say they’re open to additional “revenue.” Presumably, some of the 40 GOP lawmakers are only eyeing closing some tax loopholes and scrapping some tax expenditures, and might very well oppose an agreement that called for even modest sacrifices from millionaires and billionaires.

That said, I’m still inclined to consider this progress — modest, incremental, barely-discernable progress. The fact that 40 House Republicans are willing to say publicly that both sides of the budget ledger “must be on the table” is, for all the caveats, a minor breakthrough.

In fact, I’ve been keeping a fairly close eye on this, and yesterday’s 100-member letter is a piece in a larger picture. Sen. Jeff Sessions (R-Ala.), for example, recently said “prepared to look at” the Buffett rule. Freshman Rep. Richard Hanna (R-N.Y.) told voters in September he’s willing to talk about higher tax rates for millionaires and billionaires. In August, four far-right House Republicans participated in a joint town-hall meeting in a very conservative area, and three of the four said they’re open to additional revenue, while one said he wouldn’t rule out tax increases on those earning over $700,000 a year.

A week later, Rep. Randy Hultgren (R-Ill.) was badgered by constituents at a town-hall meeting on the need to raise taxes on the wealthy and corporations, and reluctantly said he’s open to ending oil-company subsidies and closing tax loopholes. Rep. Fred Upton (R-Mich.), confronted by 200 angry constituents the same week, said the same thing.

Maybe this is the result of overwhelming public support for increased taxes on the rich; maybe it’s the result of a sincere desire to cut the debt; and maybe some members want to give Congress’ 9% approval rating a boost. Whatever the motivation, it’s probably a step in the right direction.

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.