Chait believes that the "whole weight of the [Democratic] party’s agenda — protecting health-care reform, progressive taxation, and everything else that might be mowed down by Republican-controlled government — rests on Barack Obama’s shoulders":
Imagine you’re Romney, trying to pass some version of the Ryan plan through a Senate with, say, 51 or 52 Democrats. In 2014, Democrats will be defending Senate seats in states like Alaska, Louisiana, Montana, South Dakota, and North Carolina. All those senators will be desperate to burnish their bipartisan credentials. Romney would barely sweat the task of picking off a couple of them to support his agenda, perhaps in return for some a favor to their local industry.
In practice, then, Democrats have just one election that gives them a remotely fair chance to win: the presidential election.