DBWI: A Conservative Party and a Progressive Party?

What would it take for the two American Political Parties, the Democrats and Republicans, to ideologically sort into a progressive and a conservative party?

OTL it seemed that following Nixon's resignation the Conservatives had won: Connally was President from 73-81 after all.

But then we had 16 years of Jimmy Carter and Bill Proxmire. Tight money, deregulation, deficit hawkishness, a certain degree of foreign policy hawkishness, Carter's being somewhat public about this evangelical faith, etc.
 
Different post-Nixon labour/economic situation. That huge round of strikes in the mid-70s, and then the financial crisis epitomized by the CREDITORS TO COUNTRY: DROP DEAD headline in the Post pretty much soured the electorate on anything within the general vicinity of Keynesianism, social democracy, or even mainstream liberalism.
 
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