The center-left is feeling ambitious these days, and it’s a heartening thing to see. Anything can happen politically, but it looks at least possible that in 2021 there won’t just be unified Democratic control of Congress and the White House, but control by a much more consistently progressive party than was the case in 2009. Maybe America can finally get truly universal health care, policies that really tackle inequality, and more.

But you don’t have to be a deficit scold to suggest that progressives should be thinking about how to pay for their policies. So it’s a source of mild concern that I keep hearing that heterodox economics — specifically Modern Monetary Theory — says that we don’t have to worry about where the money will come from, that because we have a printing press deficits don’t matter.

Now, I am not a fan of MMT, which is basically Abba Lerner’s “functional finance,” which while clever missed some possibly important things. I explained all of that in a previous post. But the truth is that none of this matters much for the issue at hand. Even if you’re a committed Lernerite, even if you think that debt never matters, the sheer scale of what progressives would like to accomplish means that there will have to be tax hikes to pay for most of it.

In other words, this isn’t mainly about theory; it’s about arithmetic.

To see what I mean, consider the biggest ticket on progressives’ wish-lists: Medicare for All. This could mean different things, and if it’s basically allowing private-sector buy-in then there’s no problem. But if it means replacing private insurance with free public coverage, you need offsetting revenue.