I think there's a certain amount of grandstanding by Moody's and S&P involved here as well, doc.
And 5412, the House Dems are kind of an unknown quantity, both in terms of what Lady Skeletor is planning as well as their current caucus discipline. Like the TEA Party, some will want or need to vote against it on their own ideological grounds, but if Skeletor doesn't actually work against it, it should end up with enough Dems who have had enough already to put it over the top.
Thing is, my sense is that she's been dealt out of the bargaining process due to her own hard-line Leftist BS in the terminal stages of making the deal, so out of either spite or a sense that Boehner and the GOP would be more harmed by failure than she would be, she could queer the deal in the House, IF she still has the party discipline to make all the Dems vote against it. If she thinks that any resultant economic turmoil this causes won't stick to the Dems, she's sadly out of touch with reality, but then since we already know she really is sadly out of touch with reality, the possibility cannot be entirely discounted. While Boehner would certainly be left in a very awkward position should she do that, it would really make Reid and Obama look like the Prince of Putzes and King of Schmucks, respectively.