One of the most telling exchanges in the first Democratic presidential debate came hours into the program in the wake of a not-so-well-explained back-and-forth between Martin O'Malley and Hillary Clinton about the 1999 repeal of a 1933 bank regulation. Offering an appealing dose of clarity, Bernie Sanders burst in with his take: "Congress does not regulate Wall Street; Wall Street regulates Congress."

This, more than any narrow dispute about a specific issue, perfectly captures the difference between Sanders and Clinton.

To Clinton, policy problems require policy solutions, and the more nuanced and narrowly tailored the solution, the better. To Sanders, policy problems stem from a fundamental imbalance of political power. The issue with finance isn't so much the presence or the absence of any specific rule; it's that the system is rigged, and the fat cats are running the show. The problem with Clinton (or with Barack Obama or O'Malley or Joe Biden or any other mainstream Democrat) is that she is part of an establishment political system that is fueled by campaign contributions and entrenches a fundamentally corrupt governance model. The solution isn't to pass a smart new law, it's to spark a "political revolution" that upends the balance of power.

Sanders's theory of change makes sense in this context

Wall Street regulation was a smart area for Sanders to raise this theme, because his worldview is especially plausible in this terrain.

Hillary Clinton's Wall Street regulation plan is smart and detailed, and wins praise from serious policy-minded financial reformers. But it's also not entirely clear what to make of it. One point Elizabeth Warren has repeatedly raised about financial regulation under the Obama administration is it's often not clear how vigorously the cops on the beat are doing their regulatory job. Enforcing existing rules more stringently could, in many ways, be a "tougher" policy than passing new rules that are then laxly enforced. Under the circumstances, the question of whether the president really wants to crush Wall Street or simply to triangulate against it is actually very important.

And in fact, you can sort of imagine Sanders bringing the largest banks to heel without really doing anything. The mere fact of his election would be enough to hammer bank stocks. If he spent his lame duck months appointing prosecutors and regulators who made clear their desire to use every tool in their kit to put megabanks under maximum scrutiny, there would probably be considerable shareholder pressure for large banks to simply break themselves up. The large, diversified "universal bank" model of a JP Morgan Chase or Bank of America simply isn't very appealing in the face of a hostile political climate.

Where Sanders falls short

When one goes further afield, however, Sanders's theory of change begins to look less plausible.

In this, he is simply treading a well-worn path of presidential candidates. George W. Bush promised to be a "uniter, not a divider." Obama swore that he could heal partisan divides and deliver huge, bipartisan congressional majorities for change. Both were frustrated. Sanders's theory is different rhetorically but fundamentally similar. It posits — against all evidence — that changing the occupant of the White House will suddenly alter dynamics in Congress and allow for the passage of sweeping legislation that is currently frustrated.

There's just no reason to believe this is true. It's clearly the case that the quest for campaign contributions is a large factor in American political life. But it's equally clear that the men and women of the Republican congressional delegation sincerely believe that tax cuts and deregulation — not higher revenues and more stringent rulemaking — are the right policy for America. To get Democratic policies passed would require the banal work of recruiting dozens of talented House and Senate candidates and getting them to win elections in GOP-held districts.