A friend of mine asked me if it’s true that “undecideds vote for the incumbent 70% of the time.” Here’s what the Cook Report had to say::
[U]ndecided voters historically have broken heavily against well-known, well-defined incumbents. This has proven true on the congressional, senatorial, gubernatorial and presidential level. That’s the origin of the phrase in politics for incumbents, “what you see is what you get” — you get pretty much the percentage on Election Day that the last round of polls indicate that you will get, while the undecided vote goes elsewhere.
So I guess not, but I guess I should check with the NES dataset.