At least half the independents, and maybe as much as 1/3 of the Democrats, loathe the GOP and would never vote for a "Republican". Where I live, the GOP is about country clubs and looking down on guys who get their hands dirty. The new party has to lose the clubbers. There are a lot of voters to gain.
Beating the Democrats is another story entirely. We’re talking about primarying out Republicans in areas where they’ve already been elected, so the party name isn’t unelectable there. There will still be lots of areas where Democrats hold power. And changing the hearts and minds of the people in those areas is a tougher task. The candidate can certainly explain how he’s different from the stereotypical Republican and that he won’t vote with the party line, only with what his beliefs are.