Donald Trump's emergence as the last man standing in the Republican presidential race has prompted his critics inside the party to intensify their search for a candidate they could back as a serious third-party alternative.

Political operatives are courting donors, calling potential candidates and developing legal contingency plans for overcoming onerous ballot qualification laws.

"This is as much as anything a battle for the future of American party politics," said Republican strategist Joel Searby, who is working with conservative writer Bill Kristol, among others, on an effort to identify a third-party candidate to run in the Nov. 8 presidential election.