I think the most damaging thing is to make it such a big issue to begin with. The outcome of 2020 is unlikely to be actually heavily influenced by the debate around the issue, even if Democrats fail to find a consensus on the impeachment process.

The most important aspect to who gets elected in 2020 is going to be the candidate the Democrats release into the general, and that is going to have a lot to do with whether the party, and the candidate, focuses on a constructive and detailed policy vision, on something to vote for.

It will also be influenced by how trustworthy the candidate is going to look to reasonable voters, which means that a candidate with an Obama-esque campaign finance anatomy might have a problem, despite fluffy rhetoric.

In any case, the issue of impeachment falls into the bucket of voting against, and that strategy has profoundly failed in 2016.