Jon Chait makes the same arguments that myself and many of my readers have made against a Bernie Sanders presidency: that while what Sanders want to accomplish is laudable, eventually his methods to accomplish them include "and then the country rallies around me and we win."
The Sanders campaign represents a revolution of rising expectations. In 2008, the last time Democrats held a contested primary, the prospect of simply taking back the presidency from Republican control was nearly enough to motivate the party’s vote. The potential to enact dramatic change was merely a bonus. After nearly two terms of power, with the prospect of Republican rule now merely hypothetical, Democrats want more.
The paradox is that the president’s ability to deliver more change is far more limited. The current occupant of the Oval Office and his successor will have a House of Representatives firmly under right-wing rule, making the prospects of important progressive legislation impossible. This hardly renders the presidency impotent, obviously. The end of Obama’s term has shown that a creative president can still drive some change.
But here is a second irony: Those areas in which a Democratic Executive branch has no power are those in which Sanders demands aggressive action, and the areas in which the Executive branch still has power now are precisely those in which Sanders has the least to say. The president retains full command of foreign affairs; can use executive authority to drive social policy change in areas like criminal justice and gender; and can, at least in theory, staff the judiciary. What the next president won’t accomplish is to increase taxes, expand social programs, or do anything to reduce inequality, given the House Republicans’ fanatically pro-inequality positions across the board. The next Democratic presidential term will be mostly defensive, a bulwark against the enactment of the radical Ryan plan. What little progress liberals can expect will be concentrated in the non-Sanders realm.
So even if you fervently endorse Sanders's policy vision (which, again for the sake of full candor, I do not), he has chosen an unusually poor time to make it the centerpiece of a presidential campaign. It can be rational for a party to move away from the center in order to set itself up for dramatic new policy changes; the risk the Republican Party accepted in 1980 when Ronald Reagan endorsed the radical new doctrine of supply-side economics allowed it to reshape the face of government. But it seems bizarre for Democrats to risk losing the presidency by embracing a politically radical doctrine that stands zero chance of enactment even if they win.
The political reality is that the House, the Senate, and 24 states are under total GOP control, along with 70 of 99 state legislatures and 31 governor's mansions. Until that is fixed, even the most left-friendly president won't be able to get things done.