Let's start with the caveat that the 2020 election is still a ways away and a lot can happen between now and then. However, for the sake of this piece, let's assume for a moment that Democrats continue their leap leftward rather than abandoning it for something more middle-of-the-road. From Medicare-for-all to the Green New Deal, the far-left wing of the Democratic Party has become energized by proposals that would massively enhance the role of the federal government in American life. Getting onboard with these progressive dreams has become an early litmus test for 2020 candidates, something the White House and the Trump campaign couldn't be happier about.
The problem the Democrats are going to have in the upcoming election cycle is weighing the urge to go all-in on their desired policies with the likelihood that doing so puts their chances of winning at serious risk. Some early contenders are already using talking points about how a majority of Americans support Medicare-for-all. What they conveniently leave out is the ever so important fact that once Americans realize part of the proposal is the elimination of their private insurance, that support takes a nosedive. Many of the early entrants into the race are co-sponsors of the Green New Deal, a proposal that's full of ambition but comically light on details. Think of it more as a progressive manifesto and less as a serious policy solution to current and future challenges.
Solid liberals make up just 19% of the overall electorate , according to a 2017 Pew Research Center study. In a recent Gallup poll, 47% of Democrats consider themselves to be either moderate or conservative. The same survey showed that the liberal wing of the party is certainly making gains, but the "democratic socialism" platform being touted by the base could prove to be too much for some Democrats in swing states, enough so that it could be a deciding factor in the election. While the president's approval rating has rarely gotten above the low 40s during his two years in office, a hard move to the left by Democrats could keep those Obama/Trump voters (who helped make Donald Trump president) in the same place they ended up in 2016, not to mention many independent voters as well.
Large swaths of the electorate are discontent and looking for new ideas to problems that never seem to get solved. However, solutions that involve a massive restructuring of America, which hands the federal government exponentially more control than it already has, is going to be an awfully tough sell at a time when trust in institutions is near rock-bottom. A move like that would also offer President Trump the opportunity to make the election more about their plans and less about defending certain aspects of his job performance, which is comparable to giving him home-field advantage for the 2020 race. Democrats are in an epic struggle between what their hearts want and what their brains are likely telling them. If they go with their hearts, as they've certainly indicated they might do in the first few weeks of 2019, a second term of Trump becomes much more possible.