I often find myself falling into the habit of thinking that when Dems capitulate to Bush it is because they are pussies. But the truth of the matter may be that the bad Dems* are actually believers in Bush's policies. As Greenwald puts it:

But a large number of elected Democrats vote in favor of the radical Bush agenda for a very simple reason: they believe in it. Despite the glorious "D" after their name, their views are materially indistinguishable from the defining ones of the Bush faction on the key issues. A huge portion of Congressional Democrats are members of the corrupt, bipartisan Beltway political establishment first, and everything only follows that, and they thus embrace and support the values of that establishment.

That's why I think Taibbi has it sort of wrong: Reid and Pelosi et al. aren't weak, they actually support the policies of the Bush administration. If they really wanted to end the war they could cut off funding:

It is a lie that Congress must pass a bill to end the occupation of Iraq. The occupation can be ended with an announcement by Congressional leaders that there will be no more funding. Any proposal to fund it can be blocked by 41 senators. Bush has plenty of money for withdrawal and could be given more for that exclusive purpose. When your television tells you the Democrats need 60 or 67 senators to end the occupation, your television is lying to you.

*today's bad Dems: "The Dodd/Feingold amendment to remove telecom immunity from the bill just failed by a whopping vote of 31-67 -- 20 votes shy of the 50 needed for a passage. A total of 18 Democrats joined all Republicans in voting for immunity: Bayh, Inouye, Johnson, Landrieu, McCaskill, Ben Nelson, Bill Nelson, Stabenow, Feinstein, Kohl, Pryor, Rockefeller, Salazar, Carper, Mikulski, Conrad, Webb, and Lincoln. Obama voted against immunity [Ed. -- good for him], and Hillary Clinton was the only Senator not voting." (emphasis added)