The Democratic party’s dilemma this year is simply stated but difficult. It can take back some of Donald Trump’s voters by nominating Senator Bernie Sanders, or it can keep the recently won support of anti-Trump former Republicans (or people who formerly tended to vote Republican) by choosing almost anyone else. It cannot, broadly speaking, do both. Although it isn’t clear which course of action would be more likely to win the presidency, a party should always consider the down-ballot consequences. And there, a Sanders nomination looks like a net minus.

Although Trump is not an extremist in policy terms, he seems extreme to many people who might accept a president with the same views who talked and acted differently. He also blatantly repudiates the moderate style that has long (almost always, really) been dominant in Republican presidential politics. Most of his supporters love that, and his opponents hate it. Trump is a name-caller and uses unprecedentedly aggressive language in other ways as well. Some of his opponents cite these characteristics in condemning him and are sincerely offended by them, but they are no loss to the Republicans since they vote Democratic regardless. But others find them deal-breakers, and this rather than policy is the main reason why they’re against Trump. Although a significant source of new support for the Democrats, they cannot be taken for granted.

Sometimes political converts are quite zealous for their new party or cause. But not always. The Never Trump people who to one degree or another have left the Republican party may not be voting Democratic for long. Their votes for Democratic candidates in competitive congressional districts in 2018 are ones the Democrats cannot afford to lose. And they aren’t a constituency for Sanders.

There are two reasons for this. One is that they don’t want their country or themselves to be oppressed by the confiscatory levels of taxation and regulation that Sanders can be expected to push for as president. The other is that their disgust at gross incivility is bipartisan, and Sanders is far from a nice guy unless you’re completely on his side. His hostility to the American economic system and passion for iron-fisted government has a psychological equivalent in his arrogant closed-mindedness. Voters who are themselves civil and somewhat open-minded can easily recognize its opposite—except among the minority of politicians who are both good at concealing harshness and arrogance and wish to, a charmed circle that doesn’t include Bernie Sanders. Granted, Sanders is real. But in his case that can be a problem.

The presidential nominee need not define the whole party, of course, and freshman Democrats in previously Republican districts can signal their discomfort with Sanders to moderate voters. But that is a difficult tactic in a superheated political environment like this year’s. My sense is that Berniecrats would expect strong, not vague, support for their guy if he were the nominee. Some Sanders backers who would reluctantly vote for, say, Joe Biden in order to get rid of Trump won’t necessarily vote for “sellout” Democrats in order to keep them in office, even though they are the ones responsible for the party’s House majority. One problem for the Democrats’ more moderate incumbents in 2020 is that they are too easily, however unfairly, associated with a Democratic establishment and a “Clinton machine” that are widely detested on the left wing of the party. They cannot afford to be charged with such sympathies more often than they already are. Both the resulting need to placate left-wing Democrats and their own total opposition to Trump would, I suspect, lead non-leftist Democratic incumbents to support Sanders (if he’s the candidate) more clearly than they would prefer to. And that might lose them more than a few of the votes they won in 2018.

Dr. Frisk is a resident fellow at the Alexander Hamilton Institute. His views are his own and do not represent those of the AHI.