A conservative donor who has been scouring Republican ranks for a third-party candidate is pressing ahead with a group to get on the ballot in dozens of states. The candidate will come once the group sees that the ballot access is possible. The donor, John Kingston, a bundler and ally of Mitt Romney, said he will bankroll a ballot-access project to create a path for someone to run as another option. The effort is being called Better for America. The idea is “to do a proof of concept for everybody,” Mr. Kingston said. “It exists, there is a pathway, there...