I really think this varies a lot by state. Certainly, Michigan has had it's fair share of politicians who should be under ethics investigations but it's never happened in my politically aware lifetime (since mid 70's or so).

Here, any individual can file an "ethics complaint" against any elected official - a good friend works as an ethics investigator, and some value >99% of the filings are immediately rejected as frivolous or unsubstantiated.

Any that require interviews or collection of supporting evidence are categorized as "ethics investigations" regardless of the degree or perceived veracity. Whether or not it should be a disqualifying factor for an official to become a candidate for higher office, I really don't know, and I'd suspect my view would change dramatically depending on the actual allegations.

I'm originally from CT, so I'll agree with catiebug has to say here. I'm not sure of the exact statute detailing ethics investigations for public officials, but Gov. Rowland was the first time I'd heard of a Governor being investigated in the state.