Republicans have a Donald Trump problem, and it is worse than they think.

They think he is a phenomenon ”“ a repository for protest ”“ that is a temporary irritant. They think he’ll go away once his supporters realize that he can’t win, or that he is an embarrassment, or that he is a buffoon, for these are the critiques of those whose poll ratings don’t match his.

What they don’t realize is that Trump does not fit into the customary profile of a presidential candidate, one who puts himself forward in Iowa, and then New Hampshire and South Carolina, and, when the money or support runs out, goes back to the Senate, or the state capital, there to hope he might be summoned to be a vice-presidential candidate.

This may be the understatement of the age, but Trump is no ordinary candidate. Most of his rivals are finding the business of running for president hard. He’s having a blast. Most of his rivals are constrained by financial factors. He’s a billionaire. Most of his rivals eventually will worry that if they remain too long in a 16-person race they will look silly. Trump is not troubled by the prospect of looking silly.

What should trouble the Republicans is not that Trump leads the GOP pack nationally in some polls, or that, according to an NBC News/ Marist poll, he is nearly tied with Gov. Scott Walker of Wisconsin in Iowa and holds a substantial lead in New Hampshire. Polls come and go, and the 2012 Republican nomination struggle, where every month a different GOP contender took the lead, is proof of that.

What instead should trouble ”“ terrify ”“ the Republicans is the fact that Trump is making noises about running as an independent candidate for president if he is not “treated fairly” by the GOP. Trump gets to decide that himself. The voters of Iowa and New Hampshire will have no say.

American history is full of third-party candidates who made a difference, either in shaping the national debate, as women’s suffrage and anti-slavery candidates or even pro-segregation candidates have done, or in deciding a presidential nomination, as some third-party candidates have done.

“Historically, the argument is overwhelming,” says Ralph Nader, the consumer advocate who has mounted third-party campaigns four times. “Again and again, third parties have pioneered transformations of the political economy that the major parties took up belatedly. There’s an enormous heritage there that we now take for granted.”

In six presidential campaigns, for example, the Socialist presidential candidate Norman Thomas inserted Social Security, unemployment insurance and the minimum wage into the American conversation. All are regarded as uncontroversial aspects of American life today.

Eugene V. Debs offered a serious critique of American society and culture and polled nearly a million votes in 1912. The same can be said for the great progressive leader Robert M. La Follette, who won nearly 5 million votes in 1924 and lost North Dakota by only 5,009 votes.

Running as a states’ rights Dixiecrat in 1948, Strom Thurmond took four states, accounting for 39 electoral votes, and presaged difficult struggles over race for the next two decades. The same can be said for George C. Wallace, whose American Independent campaign of 1968 gave him five states and 46 electoral votes.

Trump, fortified with impulses rather than issues, is exceedingly unlikely to reshape the national debate, though his very presence in the race ”“ and his high poll ratings ”“ surely reflects the poverty of the national debate and the alienation of many Americans.

In that regard, Trump mirrors the campaign of H. Ross Perot in 1992, who won more than 19 million votes and, to some extent, John B. Anderson, who won nearly 6 million votes in 1980 by drawing on disaffected liberals and conservatives.

A mainstream political figure, Anderson of Illinois, who served 20 years in the House, half of them as chairman of the House Republican Conference, might have created a more enduring political force in America. At one point a Gallup poll put him at 26 percent, but he could not sustain that level of support and was unable to forge a lasting impact.

“There have been opportunities, actually, to create a real third party,” says John Savagian, a historian at Alverno College in Wisconsin, “and those have been opportunities that have been lost.”

As an independent candidate in the general election, Trump is unlikely to be the founding father of a new political movement. But he retains the potential to do what Wallace (1968) and Nader (2000) may have done, which is to affect the outcome of the election in very close contests.

In 1968, the presence of Wallace in the race may have given the White House to Nixon. In 2000, the presence of Nader may have given the presidency to George W. Bush.

This is speculative, of course, and Nader ardently questions this line of thinking. But if Nader’s 22,198 votes in New Hampshire had gone to Al Gore, the vice president would have defeated Gov. Bush there, even if Patrick J. Buchanan’s 2,615 votes on the Reform ticket had gone to Bush. Those four electoral votes from the Granite State would have given Gore 270 electoral votes, as opposed to Bush’s 267, and nobody would have worried about hanging chads in Florida.

All of this is to say that, if Trump runs as an independent and draws more from the Republican nominee than from the Democratic nominee, he could endanger the GOP’s efforts to reclaim the White House in 2017.

The Democrats may evade that kind of threat this time, as their early-campaign dark horse darling, Sen. Bernie Sanders of Vermont, has disavowed an independent campaign.

“No matter what I do, I will not be a spoiler,” Sanders said in a conference call seven months ago. “I will not play that role in helping to elect some right-wing Republican as president of the United States.”

Trump has made just the opposite threat. Before long he may have the Sanders comment put to him, with the question: “Will you be a spoiler and help to elect some left-wing Democrat as president of the United States?”

— David M. Shribman is executive editor of the Post-Gazette (dshribman@post-gazette.com, 412 263-1890). Follow him on Twitter at ShribmanPG.