The moment a third party becomes successful, all the centrist hacks and their money will come oozing in, and we’ll be right back where we started. Well, almost, because we will have invested an enormous amount of effort into what the hacks will once again take over and ceded the elections in the interim to the hard core right wing.
Better to keep hammering the centrist and sellout hacks with primary opponents, hound them with protests in their districts, and run ad campaigns from hell, while building an independent progressive power base outside the party. And unions, lots more unions. That’s what you’d need to keep the new third party candidates honest anyway.