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.

A separate group, Conservatives Against Trump, which includes blogger Erick Erickson, has been holding calls and meetings to discuss third-party candidates as well as other options to stop the New York billionaire from winning the White House.

The hurdles to a third-party candidacy are immense. No independent candidate has ever won a presidential election, although some have played spoilers. But the efforts by the Republican groups underscore the unusual divisiveness of Trump's candidacy within Republican ranks ahead of a likely general election fight with Democratic front-runner Hillary Clinton.

Trump's opposition to free trade is at odds with the views of many Republicans, especially in the party's business wing. Many of Trump's critics also find his rhetoric offensive, including his call to temporarily bar Muslims from entering the country and his comment describing Mexican immigrants to the United States as rapists and drug dealers.

Some Republicans say they worry that any third-party candidate would only siphon votes away from Trump and help Clinton win the election.

Ralph Nader's Green Party presidential run has been blamed by some Democrats for the razor-thin defeat of Democratic nominee Al Gore in the 2000 election. Ross Perot's independent candidacy in 1992 was seen by some Republicans as contributing to President George H.W. Bush's loss to Democrat Bill Clinton.

One outcome, though rare, may be that no candidate crosses the necessary threshold of 270 votes in the U.S. Electoral College. In that case, the vote for the next president would pass to the U.S. House of Representatives, currently controlled by Republicans.

Deborah DeMoss Fonseca, who recruited donors for former Republican presidential candidate Jeb Bush and is working with Conservatives Against Trump, said her group was trying to find a candidate who would be high-profile enough to compete with Trump and Clinton.

NO EASY TASK

But finding a candidate of that caliber who would be willing to run is no easy feat. Searby's group has reached out to former Secretary of State Condoleezza Rice and James Mattis, a retired U.S. Marine Corps general, among others, but both declined after discussions.

Republican U.S. Senator Ben Sasse of Nebraska has emerged as a favorite of the Republicans seeking a third-party candidate. Kristol has had warm words for him.

Sasse, a freshman lawmaker and former Bush administration official, is a strong critic of Trump and has called for an alternative candidate to him. But he says that person should be someone other than him.

On Wednesday, the morning after Trump emerged as the presumptive presidential nominee, phones at the office of Libertarian Party candidate Gary Johnson were ringing off the hook with calls from small-government Republicans who feel they cannot get behind Trump.

But as a Libertarian, Johnson holds views on some issues, such as the legalization of marijuana, that are antithetical to the views of some Republicans.

In March, former New York Mayor Michael Bloomberg, an independent, said he had considered jumping into the race as a third-party candidate but opted against it, saying he feared it would only serve to help get Trump elected.

One of the biggest hurdles to a third-party run is simply getting on the ballot in enough states to mount a viable campaign.

Texas requires more than 79,000 signatures from voters who did not participate in either primary. Its deadline is Monday. Among other states, North Carolina's deadline is the end of May, and Illinois and Florida in mid-July.

"A third-party candidate is a pipe dream," said Republican strategist Tony Fratto, who worked in Bush's administration and strongly opposes Trump. "What's going to happen is Hillary Clinton is going to win big. It won't be close."