Instead, however, he offered a plan that was clearly both too small and too heavily reliant on tax cuts. Why? Because he wanted the plan to have broad bipartisan support, and believed that it would. Not long ago administration strategists were talking about getting 80 or more votes in the Senate.

Mr. Obama’s postpartisan yearnings may also explain why he didn’t do something crucially important: speak forcefully about how government spending can help support the economy. Instead, he let conservatives define the debate, waiting until late last week before finally saying what needed to be said  that increasing spending is the whole point of the plan.

And Mr. Obama got nothing in return for his bipartisan outreach. Not one Republican voted for the House version of the stimulus plan, which was, by the way, better focused than the original administration proposal.

In the Senate, Republicans inveighed against “pork”  although the wasteful spending they claimed to have identified (much of it was fully justified) was a trivial share of the bill’s total. And they decried the bill’s cost  even as 36 out of 41 Republican senators voted to replace the Obama plan with $3 trillion, that’s right, $3 trillion in tax cuts over 10 years.

So Mr. Obama was reduced to bargaining for the votes of those centrists. And the centrists, predictably, extracted a pound of flesh  not, as far as anyone can tell, based on any coherent economic argument, but simply to demonstrate their centrist mojo. They probably would have demanded that $100 billion or so be cut from anything Mr. Obama proposed; by coming in with such a low initial bid, the president guaranteed that the final deal would be much too small.

Such are the perils of negotiating with yourself.

Now, House and Senate negotiators have to reconcile their versions of the stimulus, and it’s possible that the final bill will undo the centrists’ worst. And Mr. Obama may be able to come back for a second round. But this was his best chance to get decisive action, and it fell short.