Say that Mr. Boehner cannot count on the support of 34 of his Republicans when it comes to passing major fiscal policy legislation. That means he would need to identify 18 Democrats who would vote along with the Republicans who remained with him.
Here’s the problem: it might be hard to round up those 18 Democrats.
The reason is that most of the Democrats who remain in the House are quite liberal.
In fact, the once-powerful Blue Dog Caucus, a coalition of moderate Democrats, will have only 14 members in the new Congress. The centrist Democrats who once filled its ranks fared very poorly in the 2010 midterm elections, while others retired or were harmed by redistricting or by primary battles. Although Republicans have moved more to the right than Democrats have moved to the left in recent years, according to measures like those developed by Mr. Poole, the attrition in the Democratic Party has nevertheless contributed to moving the two parties even further apart.
What that means is that if Mr. Boehner has a significant number of Republican defections, as he did on Thursday night, he will need to win the support of at least some liberal Democrats. And a bill that wins the support of some liberal Democrats will be an even harder sell to Mr. Boehner’s Republicans. For each vote that he picks up from the left, he could risk losing another from his right flank.