[Torie]

No, but the data points are too few for this kind of thing to achieve much statistical interest. Kennedy did beat Nixon however, a quasi incumbent. Normally incumbent presidents don't lose historically absent stress, but we have a more divided politics, with fewer swing voters, since the 1876 to 1896 period.

[William McKinley]

Nothing is constant but change, and every first that has ever happened never had happened before.

But I stand by my assertion that voters tend to look at those with only legislative experience a bit more skeptically, and I see no reason to think that has changed.