I do actually think Feingold is an interesting possibility. I know people say it can't happen because of his loss in the Senate race, but Santorum got obliterated in his reelection bid in 2006, yet came in second place in the 2012 GOP primaries. Heck, Gingrich came in third that year (at least in the popular vote), despite having been out of office for 14 years and having resigned in disgrace from his last political office. In both cases, their past electoral failures were barely even brought up. The voters didn't care. I don't think many voters care about such things.
It's probably a bigger deal if you're aiming to run an "establishment" campaign, because party elites will be reluctant to back you if you haven't demonstrated electoral success, but if you're running as an "insurgent", then I don't think it really matters.
