Traditionally, third party candidates perform best in races with no second party opposition. A Libertarian or Green running against an otherwise-unopposed candidate will do 10-20 pts better than one running against candidates from both parties.

If you go back to Bush v Gore, Nader underperformed his popular approval. It was Pat Buchanan who significantly over performed in Democrat districts. And that was largely attributed to the (deliberately) poor design of the Butterfly Ballot.