The markets aren’t tanking on fears of debt. They’re tanking on fears of growth.

The Dow is down 300 points today. So should we blame the supercommittee?

Perhaps. But not in the way that you think. There’s no real reason for the market to care whether deficit reduction comes through “the trigger” or through a supercommittee deal. It’s about $1.2 trillion either way. If you’re worried about how much debt the United States is carrying around, the supercommittee’s failure changes nothing.

But from the perspective of near-term stimulus, and thus next year’s economic growth, the supercommittee’s defeat could change quite a lot. As JP Morgan’s Michael Feroli writes, “The real near-term fiscal issue is the expiration of certain temporary support measure at the end of the year, in particular the payroll tax holiday and emergency unemployment compensation. While the fate of those measures are not directly tied to the supercommittee, this week’s failure to come to an agreement probably reduces the likelihood some that those programs get extended, and thus we can expect a noticeable drag from fiscal consolidation early next year.”

Barry Knapp, head of U.S. equity strategy at Barclays, said much the same thing to Bloomberg News. “The bigger problem is that a deal in the supercommittee was expected to pave the way to extend the stimulus that is in the system. If you don’t get a deal, which is probable, you get a big hit to the economy in the first quarter right at the point when the economic fallout from the European debt crisis is hitting.”

A related issue is that the trigger’s cuts aren’t backloaded: They’re as heavy in 2013 as they are in 2019. If the economy remains weak in 2013 — and it probably will — that could be a problem. “Relative to our current forecast,” writes Macroeconomic Advisers, “a ‘sequester’ implies twice as much fiscal drag in 2013.” (See graph on right.)

So the supercommittee’s failure certainly doesn’t comfort the markets. But not because the deficit picture is very different than it would have been if the supercommittee had succeeded in finding $1.2 trillion in cuts. As Nouriel Roubini puts it in a concise tweet, “Supercommitee collapse means greater fiscal drag risk for ‘12 not greater sovereign risk: that’s why bond yields are down & stocks plunging.”

It’s also worth noting that the world is concerned about quite a bit more than the supercommittee. Interest rates are spiking on Spanish debt, for instance. So though Washington is making a mess of things, attributing the entirety of Monday’s market movement to the supercommittee is not a wise thing to do.