Kausfiles quotes James Taranto of the Wall Street Journal who gives two very good reasons why Democrats might postpone a fight over the filibuster rule:
Kaus speculates that the Republicans could force a rules change vote by voting against cloture. This, it seems to us, however, would be too cynical to be worth doing. If some Republican Senators were to attempt to force a showdown on a rules change vote by causing the cloture vote to fail, they would risk antagonizing the half-dozen or so "moderate" Republican senators. These then might, in a fit of pique, decide to vote with the Democrats to reject the motion to change the rule, and the whole thing would blow up in the Republicans' faces.
Washington politics is Byzantine, but it's probably not that Byzantine.