Having lived in this district: the issue with nominating a candidate substantially to the left of Tsongas isn't that they'd risk losing to a moderate Republican. Rather, it's simply that the Democratic primary voters in the district wouldn't want a candidate like that. As some have alluded to earlier in the thread, the Democrats here are moderate/pragmatic types. In fact, we can realistically expect a nominee less liberal than Tsongas.
Generally I agree, but I could see a progressive type breaking through in a crowded primary.
Does anyone know how liberal Koh is? I've been assuming that he's basically where Tsongas is politically, but I have no idea.
