There’s seldom reason to cheer when it comes to legislators getting at the root causes of our national debt, which is officially $14 trillion but is really north of $60 trillion when you factor in unfunded promises to the real drivers, Medicare and Social Security.

But, lo and behold, a bipartisan group of senators is working on a plan to reduce the debt by $4 trillion over 10 years. Their effort is different from the battle now under way on Capitol Hill, where legislators are haggling over $60 billion in cuts from discretionary domestic programs. These senators are looking at the much harder problem of overhauling guaranteed programs like Medicare and reshaping the tax code. So, there is reason to cheer, although we offer that as conditional praise with one big caveat.

Let’s start with the reason to cheer. The so-called Gang of Six consists of Senate Republicans Tom Coburn of Oklahoma, Saxby Chambliss of Georgia and Mike Crapo of Idaho, and Democrats Mark Warner of Virginia, Kent Conrad of North Dakota and Dick Durbin of Illinois. They run the gamut from conservative to moderate to liberal, so they touch various points on the political compass.

Their work also has mirrored that of President Barack Obama’s debt commission, whose December report offered options for entitlement overhauls, domestic spending cuts and tax revisions. Encouragingly, several senators are watching their colleagues with interest, seeing it as a way to finally get Congress moving on the larger fiscal challenges.

But here’s the caveat, and it’s a big one. Reports have surfaced at Politico.com and the Wall Street Journal that suggest some senators, including among the Gang of Six, are getting nervous about overhauling Social Security. So are some House Republicans, although Speaker John Boehner told the Journal that the House won’t flinch.

Let’s be clear: There is no way the nation can rein in that monster debt without taking on Social Security. Even though the program won’t go over the edge tomorrow, it is headed toward trouble. And Social Security’s finances cannot be fixed overnight.

What’s more, there’s no way Washington will ever reform Medicare or the tax code if lawmakers don’t fix Social Security’s finances. Punting on Social Security means they lack the will to make tough decisions. “This is like a sweater,” Maya MacGuineas of the Committee for a Responsible Federal Budget told us. “If you pull out one thread, the whole sweater falls apart.”

The senators can’t let this sweater unravel, not when you consider the generational inequity of passing along a massive debt to the next generations of Americans. The rest of the world — particularly our creditors — waits to see if we have the stomach to get our debt under control. We will never achieve that without also reforming Social Security.