The threat of recession is having one salutary effect — reminiscent of developments on Capitol Hill immediately after the terrorist attacks of Sept. 11, 2001: For a moment, Republicans and Democrats are putting aside their differences in the face of a common enemy.

Unfortunately, as we know, the post-9/11 moment did not last. Within weeks of President Bush hugging then-Democratic leaders Sen. Tom Daschle (S.D.) and Rep. Richard Gephardt (Mo.) on the House floor, Washington, D.C.’s dismal polarization had reasserted itself, blocking agreement to solve the country’s urgent problems.

Right now, Bush and Congressional leaders of both parties seem to be motivated by terror of a severe and lasting economic downturn into approaching agreement on a bipartisan short-term stimulus package. We have some reason to hope that Bush will agree to Democratic demands that it put money into the hands of people who don’t pay income tax and that Democrats will agree to GOP demands for breaks to encourage business investment.

Cynics will say, “Well, yes. When it comes to spending money to shower on people, Democrats and Republicans can always agree.” But it’s worth observing that, for the moment anyway, both sides are determined to avoid making demands that might kill the stimulus agreement — such as making Bush’s 2001 tax cuts permanent or imposing regulatory restrictions on the mortgage industry.

But can an atmosphere of bipartisan cooperation that’s born of adversity last beyond the moment? Given all recent history, we have to be deeply skeptical. There are controversial matters coming up — notably reauthorization of the Protect America Act, due to expire in nine days — which could put Republicans and Democrats at each others’ throats again.

And yet, as the Senate returned to town to start the second session of this Congress, the leaders of both parties demonstrated that they understood that whatever got accomplished in the first session owed to bipartisan agreement — and that excess partisanship blocked what didn’t.

The way Majority Leader Harry Reid (D-Nev.) put it was: “The past year made one thing clear. We in the Senate are at a constant crossroads, with two paths from which to choose. One is bipartisanship. The other is obstruction. One path leads to change. The other, to more of the same. When we chose bipartisanship, we made real progress for the American people.”

He cited ethics legislation, the minimum-wage hike, veterans health, the energy bill and student aid. Similarly, in his session-opening address, Minority Leader Mitch McConnell (R-Ky.) observed that, “Personally, I think there are a lot of lessons we can take away from last year, and if we are smart we will learn from them. We all know what worked and what didn’t work. We all know the formula for success and the formula for failure. So, this year, even more than last year, success and failure will be a choice.”

“I think we can agree … that we all worked best last year when we worked together,” McConnell said. Indeed, it’s a choice.