The single biggest problem standing between the left and sustained and meaningful control of the federal government is the complete lack of ability of voters to circle around a consensus candidate. There are several valid reasons to be critical of Harris just as there are pretty much every single Democratic Presidential decade basically of my lifetime. But Republicans vote consistently for candidates they dislike or even hate just to beat Democrats. Every single candidate for the Democratic nomination in 2016, 2020, and undoubtedly in 2028 will have some vocal subset of registered Democrat voters telling you exactly why they will never in a million years vote for them. I saw it constantly on Reddit and I don't see any reason why it won't continue.

Until somebody drops the magic "consensus candidate" name that somehow pleases everyone, Democratic voters are always going to be a major hurdle to their own success. And frankly I don't think that "consensus candidate" name exists. Such is the curse of being the big tent party opposite the GOP. Republicans know they can continue winning elections for at least a little longer thanks to Democratic infighting alone.