Doesn't have to be commercial. For example, if someone uses a pic they took of you in public as part of a political ad, and you didn't sign a release, they are liable and have to remove your image from the ad or stop running it.
I assume most would understand I mean commercial purposes to be inclusive of advertising.
The main exemptions to requiring a release are public figures (Huskey v. NBC, 1956) and newsworthy events (Time v. Hill, 1976)
When we talk about lacking a "reasonable expectation to privacy", people are referring to Forster v Manchester (1963) where the court basically stated that if you can be observed from a public thoroughfare you cannot object to surveillance. So if you're at a public range you cannot object to being photographed, even if you are not a public figure (which has no clear legal definition) and are not newsworthy (ditto).
Politicians are public figures and most of what they do is probably considered newsworthy (for better or worse), so that's why they don't need model releases to run opponent photos in attack ads.
Yes if someone took a photo of me and prominently used it as an ad "Big Blue wants to take your guns away, that's why he supports proposition 3738, so should you! Think of the children!", then I could sue. I'd have to consult a suitably experienced privacy lawyer but my suit would likely be based under either False Light and/or Misappropriation of Likeness (privacy torts).
However if I attended a rally, and happened to be in a photo (say behind Obama in some GOP ad) that was used - I likely would not have much of a case. Per Faloona v. Hustler the publisher would be liable for any damages I sustained; but given the use of the photo was not really focused on appropriating my likeness for their message, it would be very challenging for me to demonstrate any damages.
So as to whether or not a model release would be required for a political ad.. It would depend. 99% of advertising and stock it's irrelevant since no agency will buy photos without the releases anyway, including most pro-political ads which I'm sure they gather the releases from the smiling supporters.
This is not legal advice. I am not a lawyer. I do take photos.