WASHINGTON — Again?

The reigning theory in this town has been that both Republicans and Democrats, worn down and exhausted by the summer debt ceiling standoff and the ensuing collapse of their poll numbers, had returned to the Capitol eager to play nice.

But this week’s nasty showdown over a short-term spending bill demonstrates that the economy may be too weak, lawmakers too restless and the 2012 election too close for even a brief period of bipartisan comity.

A small but determined group of conservative Republicans on Wednesday again defied their leaders and brought down a bill to keep the federal lights on after Sept. 30 because it did not meet their demands to make deeper spending cuts.

House Democrats fed up by the Republicans’ past refusal to compromise and outraged over the handling of disaster aid (something some Republicans also did not like) responded with their own hard-line tactics, refusing to supply Republican leaders with the votes they needed to pass the must-pass measure, leaving the government on the brink of yet another shutdown.