I’m honestly shocked, and not just because after clips like this my expectations for fiscal sanity among Democrats are absolute zero. Two noteworthy aspects here. First, Udall may be from a purple state (Colorado) but he’s not up for reelection in 2012. He’s safe until 2014. Why he’s sponsoring this now is therefore unclear to me, unless (a) the deficit is now so politically radioactive that vulnerable Democrats are trying to get ahead of it almost four years before election day or (b) this guy is honestly committed to reining in spending on the merits. I feel like we’ve stumbled upon a unicorn grazing in the forest: Why, it’s a Blue Dog who really is serious about shrinking government. They exist.

The second reason? This is a mighty bold move with the showdown on the debt ceiling just weeks away. A balanced budget amendment is one of the core demands of allegedly “crazy” tea partiers like Rand Paul and Mike Lee in exchange for agreeing to vote to raise the ceiling. Udall’s given them some valuable leverage here in being able to claim bipartisan support for a BBA.

Sen. Mark Udall (D-Colo.) announced Tuesday that he will co-sponsor a Constitutional amendment requiring a balanced federal budget. He is the first Democrat to sponsor such a measure in many years.

Udall is introducing his balanced budget amendment bill with Sen. Richard Shelby (R-Ala.). The bill would require that spending not exceed revenue in any given year…

The Udall bill would also limit federal spending to the historical average of 20 percent of gross domestic product in a given year. It currently stands at 24.7 percent…

The Udall bill would allow Congress, through three-fifths votes in both houses, to waive the balanced budget requirement. In this way it differs from a rival GOP balanced budget bill, sponsored by Sens. Orrin Hatch (R-Utah) with 23 co-sponsors, which would require a two-thirds vote of both Houses to exceed the 20 percent cap or to spend more than is collected in revenues.

I’d prefer no waiver option but realistically there has to be a mechanism for deficit spending during emergencies, i.e. war. I’m not thrilled with the lower threshold either — three-fifths in the Senate is what it takes to pass any bill in the age of the filibuster — but it’s a reasonably high hurdle (261 votes) for action in the House. Presumably the lower threshold is Udall’s concession to his own side, a glimmer of hope to the Reids and Pelosis of the world that they can still spend the country into financial ruin even under a BBA once the golden age of the progressive supermajority finally begins. Good luck with that.

Exit question: Which others Dems will join Udall on this? Ben Nelson’s polling is in the toilet and getting behind this might help pull him out. McCaskill is also highly vulnerable next cycle, although it looks like she’d prefer to handle balancing the budget with a more prosaic statutory spending cap. She and Bob Corker are co-sponsoring a bill to limit annual spending to 20.6 percent of GDP, with across-the-board cuts mandatory if Congress fails to hit the target. It’s weaker than a BBA, but surely more legislatively feasible, needless to say.