Conservatives have been claiming that a new report by the Congressional Budget Office shows that the Affordable Care Act will cost $115 billion more than estimated, because of discretionary budget costs. (That refers to money Congress appropriates every year to run agencies, as opposed to checks the government writes for benefits.)

Megan McArdle leaps all over this:

The progressive response on this, as I understand it, is threefold: 
1. We don't have to fund this stuff

2. Maybe we'll cut something else to fund this stuff
3. C'mon, who cares?
Predictably, I find none of these convincing.

That "as I understand it" turns out to be a key qualifier, because McArdle does not understand the progressive response at all. The progressive response, courtesy of Ezra Klein, is that this figure does not represent new costs of the Affordable Care Act at all:

[T]he majority of the money in the CBO's estimate -- $86 billion, to be exact -- was for existing programs. "For example," writes CBO director Doug Elmendorf in a post meant to clarify the report, "those potential costs include $39 billion authorized for Indian health services that already receive appropriations every year." That is to say, we'd be spending that money with or without the Affordable Care Act. To make this a bit clearer, I broke the $115 billion figure out into a graph:

It's funny how much more persuasive your arguments are when, instead of quoting your opponents claims before refuting them, you get to make up your opponents' claims for them.