Win, lose or draw on Tuesday, the Democratic Party will almost immediately turn its focus to the next presidential election and the fight between the establishment center and the left wing. But while the Democrats have that argument, they must also undertake the far more important task of thinking about what they agree on, and how they can construct a story about how the economy works and grows and spreads prosperity, a story that competes with — and defeats — the Republicans’ own narrative.

For 40 years, with a few exceptions, Democrats have utterly failed to do so. Until they fix this, they will lose economic arguments to the Republicans — even though majorities disagree with the Republicans on many questions — because every economic debate will proceed from Republican assumptions that make it all but impossible for Democrats to argue their case forcefully.

Republicans have a theory and a story about how the economy grows. You know it as supply-side economics: Cutting taxes, especially on the rich, and decreasing regulation will unleash so much innovation and economic activity that tax revenues will actually increase and the entire economy will benefit.

This has been the conservative story, which the right has elevated to veritable religion, for 30 or 40 years now. And the Democrats’ alternative story is … what? If you’re not recalling it, that’s because there isn’t one.