Can Donald Trump be stopped short of the Republican nomination? Probably not. But the rational wing of the party has to try — quickly and forcefully — to make that happen.

The stakes are far too high for the rationalists to stay on the sidelines, and their first motive should be political self-interest.

“I think Trump would be a disaster,” says strategist Stuart Stevens, speaking for many pragmatic Republicans. With The Donald heading the ticket, pragmatists fear, the GOP could not only lose the White House, but also the Senate.

But there is a deeper reason, beyond partisanship, to stop Trump: He is one of the least qualified candidates ever to make a serious run for the presidency. If he is nominated by a major party — let alone elected — the reputation of the United States would suffer around the world.

He would not “make America great again.” He would make America weak again. He would not increase our power and influence; he would degrade it. That’s why the national interest requires a maximum effort to thwart Trump now. And while the task will be difficult, it is not impossible.

Trump’s performance has been remarkably consistent. He received 32.5 percent of the vote in the South Carolina primary (but 45 percent in Nevada). According to Real Clear Politics, his average in national polls is 34.2 percent.

In a fragmented field, that’s been enough to win. But what happens when the field narrows? Sure, Trump will attract some voters who supported other contenders, but he’s been a prominent national figure for many years. People have had plenty of time to consider his credentials, and two-thirds of Republicans have consistently rejected him.

Look at South Carolina. Late deciders went heavily for other candidates; most Trump backers made up their minds months ago. That indicates he has both a strong following and a significant ceiling. So how can his opponents solidify the anti-Trump vote?

Clearly they don’t have many weapons. They cannot cut off campaign funds, because he is mainly spending his own money. And through his skillful use of TV and social media, Trump has effectively created his own platform, The Trump Network, to reach his followers directly and establish a campaign organization in cyberspace.

Marco Rubio is already attracting many donors and party leaders who had backed Jeb Bush, but those Bushies have painfully demonstrated that their checks and endorsements have limited value this year.

That leaves only one way to block Trump: Convince enough voters that he would be a dangerous choice, both for the party and the country. One-third of the GOP seems immune to those blandishments, but that leaves plenty of other targets, and here are three lines of argument that might work.

One: Trump cannot win in November. Only 15 percent of South Carolina Republicans cited electability as their main motive, but of those voters, 4 out of 5 chose someone other than Trump.

In a recent AP poll, 60 percent of all registered voters expressed an unfavorable view of Trump and 54 percent said they would definitely not vote for him. That’s hardly a great way to start a campaign, with a majority dead-set against you.

The death of Justice Antonin Scalia gives the anti-Trumpists added ammunition. Elections have consequences. Almost certainly, the first job of the new president will be to pick Scalia’s replacement, which will affect the court’s balance for a generation.

Two: Trump’s policies are deeply flawed. Some are totally unworkable and profoundly cynical, such as throwing out undocumented immigrants.

Others are truly damaging, such as igniting a trade war with China or barring Muslims from entering the country. That would hand ISIS a public relations coup and alienate the very allies we need to fight the jihadists.

Three: Trump lacks the character and temperament to be president. Princeton political scientist Fred I. Greenstein studied the last 12 presidents, and concluded that “emotional intelligence” was the most important quality in determining their success.

“Beware the presidential contender who lacks emotional intelligence,” he wrote. “In its absence, all else may turn to ashes.”

Americans want and need a president who will keep them safe and secure, who will meet a crisis with calm judgment and clear vision. Nothing in Trump’s background — absolutely nothing — remotely qualifies him to be that kind of leader. And his late-night tweets, often unfair and unhinged, only aggravate concerns about his stability.

That’s why Republicans of good will and good sense must try to stop him.

Steve and Cokie Roberts can be contacted by email at stevecokie@gmail.com.