There are three major parties in U.S. politics: Democrat, Republican, and None-Of-The-Above (e.g. not voting). So really there are three types of swing voters: N-D swing voters (left of democrat), D-R swing voters (“moderates”), and R-N swing voters (right of republican). In competitive races, the None-Of-The-Above voters are the ones who see Democrats and Republicans as so similar — and unpalatable — that it isn’t worth jumping through the hoops necessary to vote for one or the other.
The far right has achieved its electoral success through a combination of making it harder for N-D swing voters to vote, and mobilizing R-N swing voters with more and more extreme right wing Republican candidates. Sure, a few D-R swing voters swung D in this election. Those are the swing voters the far right could afford to lose, for the sake of mobilizing the R-N swing voters. So the Democratic Party’s strategy of compromising with moderates at the expense of the base (the margins), is a losing strategy.