In the late 1940s, a bureaucrat in the Bureau of the Budget (the predecessor to today's Office of Management and Budget) came up with what would come to be known as Miles' Law. When one of his subordinates took a job in another agency, Rufus Miles predicted that the gentleman would soon change his position on the cuts to that second agency's budget that he had theretofore advocated. Miles was right, giving rise to the maxim, "Where you stand depends on where you sit." Once the man's direct interests changed, so would his beliefs.

With the first great legislative drama of the Trump administration underway, it's natural to think that this is a fight over ideology. How much responsibility to ensure health security does government have? Is health care a right or a privilege? Should the free market be left to do what it will even if it fails? These kinds of questions do matter a great deal in the design of competing plans for our health-care system. But the practical question of the moment — is the Republican effort to dismantle the Affordable Care Act going to succeed or fail? — will be determined by where everyone sits.

Let's take the illustrative example of Tom Cotton, a spectacularly conservative senator and future presidential candidate. Known for saying things like "Groups like the Islamic State collaborate with drug cartels in Mexico ... They could infiltrate our defenseless border and attack us right here in places like Arkansas," Cotton is a darling of the Tea Party right, with a 96 percent lifetime rating from the American Conservative Union.

So you might expect him to either support the ObamaCare repeal bill written by Paul Ryan and supported by the Trump White House, or, if anything, attack it from the right. Yet Cotton has sounded like the voice of cool reason, counseling care, patience, and a rewriting of Ryan's bill. "As it's written today, this bill in the House of Representatives cannot pass the Senate," Cotton says. "And I believe it would have adverse consequences for millions of Americans. And it wouldn't deliver on our promises to reduce the cost of health insurance for Americans."

You might have expected to hear that kind of thing from one of the few moderate Republicans left in Congress, like Susan Collins of Maine. But why Cotton? When you learn what effect the Affordable Care Act has had in Arkansas, it makes a lot more sense. It may be a heavily Republican state (President Trump beat Hillary Clinton there by a 27-point margin), but it accepted the ACA's expansion of Medicaid, and the results were transformative. According to Gallup polls, in 2013 nearly a quarter of the state's residents (22.5 percent) were uninsured. By 2016, when the ACA had been fully implemented, that figure had dropped to 10 percent. Only Kentucky — another red state that accepted the expansion — saw as dramatic a reduction in the uninsured. If the ACA were repealed, 381,000 of Cotton's constituents, or nearly 13 percent of the state's population, stand to lose their health coverage. Allow that to happen, and his re-election doesn't look so certain.

I have little doubt that in his heart, Tom Cotton would like to see the ACA cast into the Lake of Fire and replaced with some glorious free-market system. But for now, he's going to do what his interests dictate. Ideology is important, but not more important than where you sit.

And guess what: That's fine. We like to tell ourselves that what we really want are politicians who are unbending in their principles, guided by philosophical tenets they will refuse to violate no matter what direction the situation of the moment might push them. But that's not what we actually want. We want them to be principled so long as they're doing what we want. We're happy to have our representatives knuckle under like cowards, as long as they're knuckling under to us and not to somebody else's interests.

That isn't to say that ideology is irrelevant, just that it can only express itself within the constraints of one's interests. The members of the House Freedom Caucus have many of the same beliefs as Tom Cotton, but their criticism of the GOP health plan is that it isn't cruel enough: They want to see it roll back the ACA's Medicaid expansion as quickly as possible and offer little, if any, help to people who can't afford coverage. But unlike Cotton, who represents an entire state, most of them come from conservative districts where their only political threat would come in the form of a primary challenge from the right. So their interest lies in taking the maximally right-wing position on anything, which happily is where their ideology leads them anyway.

Nevertheless, nothing is predetermined. The interests of a member of Congress can change over the course of a debate, if it looks like his constituents have changed their minds or become angry and energized. That's what the "Indivisible" movement is trying to do: not only persuade citizens to oppose the ACA repeal effort (and the rest of the Trump agenda) but convince members of Congress that supporting it represents too much of a risk. Those raucous town hall meetings are meant to affect the running calculus every politician keeps in their head of the risks and rewards of taking a particular side.

And nobody's interests are more malleable than President Trump. He's completely unconstrained by ideology, wondering only what's good for his own popularity. Right now he faces a dilemma, because both courses on the ACA are fraught with peril: Fail to pass repeal and the base will be upset, but succeed and the results will be awful, leading to an inevitable backlash. It's clear he knows virtually nothing about the content of health-care policy, and doesn't really care.

But if turns his back on the Ryan bill — which he just might — it'll be because he decided it was no longer in his interest for it to pass. And Miles' Law will hold yet again.