Donald Trump looks like the clear Republican front-runner. At the same time, there is a growing #NeverTrump movement, with Republican, conservative and libertarian officeholders, media personalities, and voters vowing that they will not vote for Trump if he is the nominee.

In fact, polls suggest that roughly half of those Republicans who do not currently back Trump would not support him if he won the nomination. Such numbers tend to shrink as the general election draws nearer and the partisan choices become starker, but there is no doubt that many more Republicans than usual are prepared to bolt the party rather than support a vulgar charlatan who flirts with bigots and casually urges Americans to commit war crimes.

But if Republicans don’t want to support Trump, what are their options?

A very few might grit their teeth and vote for Hillary Clinton. But one suspects that a dishonest left-winger, who doesn’t think the Obama administration is liberal enough, would not be a palatable choice for most.

Could they really vote for her knowing that she would, for example, most likely appoint the next Supreme Court justice? Many more might just stay home, but that would not only lead to a Clinton victory, it would almost certainly guarantee Democratic control of the Senate and possibly even threaten the House.

Some have talked about a more conventional scenario in which a conservative mounts a third-party challenge, but the barriers to such a run are enormous.

Potential candidates would have to meet petition-signature requirements, with filing deadlines as early as mid-August. In California, the candidate would have to get signatures equivalent to 1 percent of the total number of registered voters, which could be roughly 178,000. Oklahoma would require signatures equivalent to 3 percent of the total votes cast in the last general election. Any third-party effort would be extremely costly and require an organizational infrastructure that few minor parties have.

The potential third-party candidate most often discussed, former New York mayor Michael Bloomberg, announced he will not run, sparing us from yet another big-government champion of the nanny state. But it is unlikely that anyone without Bloomberg’s billions would be able to mount a credible independent run.

That would leave existing alternative parties and their candidates. The Libertarian party was on the ballot in 48 states and D.C. in 2012; it is already on the ballot in 31 states, and could conceivably end up getting on the ballot in every state.

The Libertarian party has never had much electoral success. Its high-water mark was 1980, when its candidates, Ed Clark and David Koch, received a bit more than 1 percent of the vote. In 2012, just under 1.3 million Americans voted for the Libertarian nominee, former New Mexico governor Gary Johnson, compared to almost 61 million voting for Mitt Romney.

Johnson is the front-runner for the Libertarian nomination again this year (there are a handful of less-well-known contenders), and may provide an attractive choice for those seeking an alternative to Trump and Clinton.

As a Republican two-term governor of New Mexico, Johnson built a reputation as a fiscal hawk, vetoing more spending bills than any other governor. In the face of a Democratic-controlled legislature, he still cut taxes 14 times, without a single tax increase. When he left office, New Mexico was one of just four states with a budget surplus. He could be well positioned to contrast his position on cutting government with the big-spending agendas of both Trump and Clinton.

On the other hand, Johnson’s more open views on social issues and his anti-interventionist foreign policy may be tough to swallow for Republicans who have already rejected Rand Paul.

Social conservatives might turn to the Constitution party, which is already on the ballot in 16 states this year, and could qualify in more.

The Constitution party, founded by Howard Phillips 25 years ago, has never received more than 0.19 percent of the vote, and the candidates competing for its nomination, Scott Copeland and J.R. Myers, are lacking both money and name recognition. Even so, its hardline positions on issues such as immigration, abortion, and gay marriage might attract Ted Cruz supporters, if Cruz does not take the nomination away from Trump.

The reality is, of course, that neither the Libertarian party’s nor the Constitution party’s candidates — or for that matter any other third-party candidates — have any real chance of winning even a single state. But if enough unhappy Republicans (and possibly a few unhappy Sanders Democrats) vote for one of them, it could throw a significant monkey wrench into this year’s election.

A Libertarian party, for instance, that takes 3 or 4 percent of the vote is very different from one that takes 1 percent. It certainly could move states from Trump to Clinton or vice versa. Recall that many believe that Ralph Nader’s Green-party bid is what shifted Florida to George W. Bush in 2000. At the very least, it could further fracture the Republican party.

Yet, even without a viable alternative, the prospect of Trumpism may be just too much for believers in liberty and limited government to swallow. In which case, an already unprecedented political year could become even more interesting.

Michael Tanner is a senior fellow at the Cato Institute and the author of “Going for Broke: Deficits, Debt, and the Entitlement Crisis.” Send comments to [email protected].