I wouldn't say it's impossible, given how incredibly weak the 'top tiers' are at this point. There's really not THAT many issues that would hurt him with the 'base', honestly. Republican voters might have been overwhelming pro war in 2008 but they're growing weary of that by now, and most of the issues Johnson takes a libertarian stance on personally he's also pro states rights on... Plus Obama's economic policies have increasingly become the focus of rage and he has far more credibility on things like the bail outs than people like Romney or Palin who supported them (Huckabee didn't, but even ignoring the tax issue Maurice Clemmons pretty much kills his chances IMO). If anything his main problem is probably name recognition more so than anything else.