Democrats only put forward a slightly more leftie candidate either after they lost an election or after they came very close to losing one. Republicans haven't put forward a more leftie candidade ever, even when they lost elections.

Sure, in a theoretical America were 80% of Americans were unshakeable convict lefties, it would make sense that both parties turn Left if they lost too many votes because that would be were 80% was that those people, being unshakeable in their political convictions, would not buldge from them.

However the Left in America is but a small minority and both parties have decades of actual proof that it's perfectly possible to keep the Left small with the right kind of propaganda because that's what they've done, again and again and again - the American political "center" isn't way to the Right of that in most of Europe by chance and wouldn't be moving even more to the Right not just in America but everywhere because people's political convictions are unshakeable and unchangeable.

Further, if there is one thing Trump has proven is that it's absolutelly feasible to move a huge fraction of voters even more to the Right when they were already very much into the Right (i.e. from Reaganism to pretty much Fascism).

Your entire theory is anchored on the idea that the electorate doesn't move, it's the politicians who move, when everything in History and even Present day, not just in Politics but even Marketing, not just in Democracy but in Authocracy, shows that the vast majority of people are incredibly easy for those who have control over a suficient fraction of the Press to push in the direction they want them to go.

(FOX News would not have the influence it has in American politics if people's political convictions were unshakeable).

Add that factor to your thinking and it makes a lot more sense that the crooked politicians looking for a payout from doing politics for very rich people would favour using propaganda to convince people that doing what's best for the rich is the right thing to do or that the fault of the problems in America is entirelly of immigrants so all the real solutions are around immigration ("and ignore all the tax cuts for the rich and deregulation I'm doing over here") rather than moving to were people were politically and doing what people wanted instead of what's best for the rich - the propaganda option works and guarantees that the mainstream politicians who chose to shift the electorate with propaganda instead of moving to were the electorate is, keep leaving politics far more wealthy than they came in.

Look around at Politics in American since at least Reagan and what I described in the paragraph above is exactly what has been happenning and is still happenning.