These are the most un-liberal Democratic members of congress, based on their voting records as measured by their DW-nominate combined scores (Combi := 3*Nom1 + Nom2). Specifically, these are the ones on the wrong side of the dashed line in the scatter plot below.
Of these, four have a good excuse, since they represent districts that are extremely vulnerable to a challenge from the right (namely the ones marked “†††” in the table). Another two represent definitely competitive districts (“††”), and another one represents a district that is at least somewhat competitive (“†”).
There is no excuse or explanation for Cuellar or Costa. They're getting primaried, and deservedly so.
There's also not much excuse for Van Drew. His past voting record is far, far out of step with his constituents. As a separate issue, lately he's been a vocal opponent of impeachment. He figured out that he has little chance of winning the D primary in 2020, so on Dec. 19th (the day after impeachment) he switched parties. He hasn't yet figured out that he has zero chance of winning the R primary. So either way he's toast, and deservedly so.
Van Drew and Peterson were the only Democrats to vote against both articles of impeachment. Golden voted for article I and against article II. Amazingly enough, Tulsi Gabbard didn't vote; she ansered "Present" on both articles. Conservative ex-Republican Amash voted for both articles.
Keep in mind that PVI is measured relative to the country as a whole -- which leans toward the Ds. So the median competitive district is about R+5. The closer it is to R+5, the more competititive it is.