In politics, if you demand a mile, you get a foot; demand a moderate inch, and at best, you get a centimeter.This is, in a nutshell, the common knock against Hillary Clinton from supporters of Senator Bernie Sanders - that in not being willing to move very far left of where the nation currently stands, that Mrs. Clinton will be unable to move the needle against recalcitrant Republicans in Congress far enough to make things better for the public.
Christopher D. Cook "The Pragmatic Case for Bernie Sanders"
But in the end, this isn't a discussion about the best way to engage in politics - it's a discussion of the best way to deal with the Republican Party. Those are different things. For supporters of Senator Sanders, the calculus is simple: pull as far away from the Republicans, policy-wise, as you possibly can, and the point where they meet you will be on the your side of the status quo. But for supporters of Mrs. Clinton, the calculus is just as simple, if you pull as far away from the Republicans as you can, they simply decide not to come to the table, and the status quo stays where it is. And so the question comes down to, if there are enough Republicans in Congress that they're going to be a serious obstacle to reform, do they have to make deals with any Democratic administration?
For the Senator Sanders camp's view to be correct, the answer to that has to be "Yes." So if a President Sanders demands a mile, the Republicans will have no option. While they'd rather not give him anything, a foot is the absolute minimum that they can offer. And so then that leaves you with a question - if the very voters who would put Senator Sanders (or Mrs. Clinton, for that matter) into office are going to leave Congress in Republican hands, or at least leave enough Republicans in the body that they could effectively block legislation that they didn't like, who is going to lever them into agreeing to give the foot?
And this, I think is what is driving Mrs. Clinton's camp to see incrementalism as the way to go. Not because it's desirable, but because asking for an inch and receiving a centimeter (which, incidentally, is much greater proportion of an inch than a foot is to a mile) allows for progress, but doesn't make it automatically better for the Republicans to simply walk away from the proposed legislation.
It's difficult to push through lasting change, even with persistent pressure, if you're opposed by someone who isn't answerable to you. And as American politics becomes more divided, politicians don't feel the need to answer to the other party's voters. This was one of the issues pointed out by Republican critics of Governor Mitt Romney's "47%" remark - the fact that he was openly saying that there was a vast swath of the American public that he was unconcerned with.
There is also a question of the Democratic electorate that Clinton V Sanders is hashing out. Which is larger: the coalition of the Left and the Center, or that of the Left and the Far Left? It's an important question, as either the Center or the Far Left will likely find the policy proposals that one of the Democratic contenders would put forth (should either of them become President) not to their liking. And of course, this plays into the question of Congressional Republicans as well - can the Far Left sweep enough of them from power to support their candidate, or can the Center actually pressure the Republicans to make concessions to their candidate?
Either way, arguing over whether Sanders' Radicalism or Clintonian Moderation will be the best policy without actively talking about what the Republicans will do - and what cards they will hold, is simply a variation on "Who would you rather have a beer with?" Because to really understand which policy is more likely to succeed, you have to understand the obstacles it will, or will not, face.