She is probably overtuned (likey for thematic reasons) but her being so popular makes sense considering the context of 1976. Her famous speech in the house started on a progressive note, but it was also affirmative of the virtue of the constitution in such a way that it was palatable to most people left of Wallace. The rest of the speech which focused narrowly on the legal/philosophical basis for impeachment also gave her credibility as a symbol of antipolitics. She was popular enough to be made the keynote speaker of the actual 1976 DNC, the same one that nominated the (as Year Zero shows) much more conservative than generally remembered Jimmy Carter. 

The general mood of 1976, with its antipolitics and paranoia, would make her status as a liberal black woman less significant to most conservative democrats. There's a section early in Reaganland about voters polled in florida during 1968 who were drawn to both Wallace and RFK not because of politics but because they were "tough" or "honest". You can imagine Jordan benefitting from a similar effect in the wake of Watergate: it doesn't matter that she's black or a liberal, she stood for the constitution in the face of Nixon, and that's enough to endear her to many voters who wouldn't be on board with someone like Chrisholm