by Jamelle Bouie

It's official: Nebraska senator Ben Nelson wins the award for most likely to vote against his party:

The Nebraska lawmaker supported his fellow Democrats last year on just 54 percent of so-called "party unity" votes—those in which a majority of Democrats opposed a majority of Republicans. That's according to Congressional Quarterly's annual vote study, which was released Monday.

Nelson's score easily put him atop the party disloyalty list for senators from either side. The runner-up, Sen. Evan Bayh, D-Ind., still voted with his party 68 percent of the time. Nelson also supported President Barack Obama just 75 percent of the time on votes in which the administration took a position—much lower than any other Senate Democrat.

Earlier this week, I called Obama a fairly mainstream liberal, which sparked a little disagreement in the comments. As it stands, I think my description is correct: on nearly everything, Obama falls squarely within the liberal mainstream. His foreign policy is interventionist, his economics are Keynesian, and he appreciates the power of markets. He supports a stronger safety net, cares about deficits, and wants to use government to correct for market failures.

That said, when it comes to actually crafting legislation, Obama's ideology (and the composition of his administration) is less important than certain pivot points in the legislature, like the senator from Nebraska, for example. Not to rehash old battles, but the president's willingness to sacrifice the public option had more to do with Ben Nelson's mercenary behavior than it did with Obama's insufficient liberalism. Likewise, the moderate size of the stimulus was a preemptive reaction to the fact that Ben Nelson—and others—would oppose a larger bill out of some gutless "centrism."

This isn't to excuse Obama's failures, but to push back against the idea that the president is the dominant player in any given circumstance. The federal government is swarmed in pivots and veto points, and what we see doesn't always tell the whole story.

Obama intends to issue an executive order that would formalize indefinite detention without trial for detainees at Guantanamo Bay. That is a dramatic reversal of candidate from Obama's position in 2008. But has Obama embraced his inner Dick Cheney? Is he responding to congressional opposition, bureaucratic challenge and intra-White House opposition? Or is it some combination of the everything? Even in national security, a realm of relative presidential freedom, Obama is constrained by other actors and institutions.

Admittedly, this is a little banal. But the Green Lantern Theory of Presidential Action still holds a lot of sway, and as such, it's always worth noting the very real limits to presidential action when the opportunity presents itself.