You don't really see vague umbrella parties without clear agendas as much in systems which have more effective electoral systems than the US's FPTP system. In an FPTP system, such parties are natural, since once you have established a position as a major party, you can win by the other side being disliked more than you, even if you aren't people's first choice (because the incentives in the electoral system are to vote for the least offensive of the major parties.)