score:4
Hmm. That's a tricky one for many reasons, which I'll hope to elucidate:
One bit of "low hanging fruit" you may want to explore first is taking a look at publications such as Time, Life, and Newsweek at your local library. While you're unlikely to find every gubernatorial, Senate, or House candidate there, you'll likely find pictures of some of the major players. In fact, in the instances where there's a local magazine available (The New Yorker springs to mind) you might even be able to drill down a bit more.
Wikipedia is awfully good with some of this stuff. For instance, pictures of the 1974 North Dakota Senatorial candidates:
http://en.wikipedia.org/wiki/United_States_Senate_election_in_North_Dakota,_1974
Just as important as getting a lot of digital pictures from there, I think, is sleuthing the sources to see where the photos came from. For instance, the North Dakota page led me to two sources: the Library of Congress and the Image Archive for the US Department of Defense.
Individual colleges might store voter information pamphlets by state. For example, I found this archive for California:
http://library.uchastings.edu/research/ballots/ballot-pamphlets.php
I would not expect all states to have .pdf copies of these available, of course.
In addition to colleges, you can try the offices of the Secretary of State for each state. For instance, I found that Washington state has a digital archive here as well as a searchable database of non-scanned items which you can request. Some of these requests could cost money but, well, I didn't say it was going to be easy.
Microfiche seems like it'd be a good #1 option but in my experience it's pretty terrible at capturing pictures, and the cameras used to take snapshots usually produce barely legible type, let alone a decent picture. Even if you could find a way to get prints off of them, it's going to be problematic to find regular issues of papers in enough locales to be able to get, say, all gubernatorial candidates between 1920 and 1980. In my own research on New York in the 1880s, for example, I had to more or less go to the New York Public Library to find news accounts of a figure I have/had been writing a book about, and at that I didn't get all the info I wanted (and had to supplement my search by going to the NYC Historical Society).
Actual, physical newspapers generally aren't going to be available much anymore, although a local library might have decided to bind them at some point or other. That takes up a lot of space so I would not expect many libraries to do this.
Good luck on your work! I imagine it's not going to be easy.