It’s not complicated. The reality is that most Americans just aren’t socialists. They’re not fascists either–at least not yet–but voting blocks that have been reliably Democratic for the last fifty-ish years (since the GOP adopted the Southern strategy) are only in the party by default. Black families have historically been relatively conservative, as have Hispanic families. That the Democrats are left of the only other major party only makes it the party of Marxists and anarchists for want of an actual left party, since the alternative for the young generation of leftists is to essentially not participate in the political process at all.

It’s a consequence of first-past-the-post elections and the way campaigns are funded, but the result is that the Democratic party includes elements that are genuinely right of center, but it can’t risk alienating them because right of center represents the majority of the country’s population. If the Democrats were to lose the Marxists on its far left flank to a true left party, the GOP would just win every national election. Similarly, if the Dems lost its conservative labor wing in the Rust Belt, same outcome. In a parliamentary government, or if FPTP were eliminated, those two wings would be separate parties and form a coalition to govern (and the fascists on the right of the GOP along with more traditional conservatives like Romney would similarly be separate parties). Since we don’t have that, the two parties basically have to try to stay as close to the ideological center within their own caucuses as possible, because any deviation in either direction hemorrhages voters and loses elections.