The primary purpose of the Democratic party is not to oppose conservative Republicans, but to oppose the far left. It has been stated that the Democratic party is where progressive legislation goes to die.

As the Democrats abandoned their base in unions and the working class they began competing with Republicans for corporate donors. To sell themselves to those donors they had to demonstrate fealty to market based capitalism, effectively abandoning their core values of public purpose in favor of the market as the preferred methodology in organizing the society.

This, of course, meant also abandoning the New Deal that FDR had made so popular, so it was never to be disclosed that the markets depended upon the federal government's ability to create money at will. If the working class ever discovered that their suffering was unnecessary and only served to supply compliant workers to business the exit from the party's voting rolls would be massive.