“His baggage is too heavy, too controversial,” says Rasmussen, 70, who was Denmark’s prime minister for most of this century’s first decade.

Yet Rasmussen, a right-of-center politician who is now a white-shoe international consultant, remains scared of Trump. What disturbs him more immediately than the idea of Trump back in the White House is a far likelier scenario: Trump winning the Republican presidential nomination.

It may seem counterintuitive to fear Trump’s nomination more than his return to power, a less probable but vastly more dangerous outcome. But Rasmussen’s mind is on the war in Ukraine — and what Trump’s candidacy might do to sabotage it.

The former NATO chief serves as an adviser to the Ukrainian government and recently came to Washington to see members of Congress and Biden administration officials. He is lobbying them to supply more and heavier weapons and to make long-term security guarantees to Ukraine.

That’s where the Trump angst comes into play.

Just by winning the Republican nomination Trump could shatter the bipartisan front in favor of Ukraine, Rasmussen fears. Trump has been forthright about his views of Russia’s invasion, praising Putin as a clever strategist in the early days of the war and recently suggesting that Ukraine should have ceded “Russian-speaking areas” in a deal with the invader.

Rasmussen says Trump’s apparent Ukraine policy would amount to “surrender.”

“I call it a geopolitical catastrophe if Trump were to be nominated, because in the campaign his influence would be destructive,” Rasmussen says. It would move Trump’s terrible ideas closer to the mainstream and make it harder to secure congressional support for the war.

Already, he notes, opinion polls show “a weakening of the support for Ukraine” in the United States. Trump’s nomination could accelerate that, Rasmussen argues: “The mere fact that his thinking appeals to a certain element, a certain segment of the American public, will push American politics in the wrong direction.”

“I really hope that Republicans will get their act together,” he says. “I do hope, I would say not only from a European perspective but from a global perspective, that Republicans will nominate a candidate that is much more attached to American global leadership than Trump and Trumpists.”

There are only a few candidates circling the Republican race who fit that description. The most promising may be Mike Pence, the former vice president who has called for aiding Ukraine extensively and denounced “apologists” for Russia in his own party. Nikki Haley, the former United Nations ambassador, has endorsed giving Ukraine all the weaponry it needs and describes the war as a fight for freedom. Neither is polling in the double digits right now.

Trump’s top rival on the right, Gov. Ron DeSantis of Florida, has echoed him on Ukraine, denouncing what he calls Biden’s “blank-check policy” of generous aid and saying that the fate of Ukraine’s border regions is not an important American concern. This week he described Russia’s savage war of aggression as a “territorial dispute.”

That “blank check” language has become a go-to formulation for Republicans (sometimes including Haley) who want to keep some distance from the war without going full Trump. The catch phrase is about the extent of House Speaker Kevin McCarthy’s stance. When he declined an invitation to Ukraine this month from President Volodymyr Zelenskyy, McCarthy said he did not need to travel to confirm he “won’t provide a blank check for anything.”

It is not clear what that means as a matter of policy, which is not exactly reassuring for Ukraine and its allies.

It is not that the Republican Party lacks committed defenders of Ukraine. There are plenty, but like Senate Minority Leader Mitch McConnell they have tended to speak softly and carry a big omnibus spending bill. An unbending Ukraine hawk, McConnell assured European leaders at the Munich Security Conference last month that Republican leaders value a “robust trans-Atlantic alliance” — whatever other raucous voices in the gallery might say.

“Don’t look at Twitter — look at people in power,” McConnell told them, listing influential House and Senate committee chairmen who have locked arms with Ukraine.

The problem is that there is not much in the last decade of Republican politics to indicate that senior legislators can be relied upon to stick to their principles when the most strident factions of the G.O.P. base are moving in another direction. And when it comes to the war, right-wing Twitter bears an uncomfortable resemblance to real life.

There is a pronounced partisan gap on Ukraine: A Gallup poll published in February, around the anniversary of Russia’s invasion, found 81 percent of Democrats wanted Ukraine to reclaim its lost lands even at the risk of drawing out the war, versus 53 percent of Republicans. Only 10 percent of Democrats believed the United States was doing too much to back Ukraine, while almost half of Republicans thought American support had gone too far.

These are consequences of letting people like Trump and Tucker Carlson, the Fox News personality who is the Ukrainian government’s most caustic American antagonist, become the loudest right-wing voices on the most urgent security issue of the day.

That may be why Rasmussen and some other center-right foreign leaders have taken it upon themselves to make a case to the American right in favor of the war.

Most visible has been Britain’s Boris Johnson, the former Tory prime minister who is pressuring governments on both sides of the Atlantic to supply fighter jets to Ukraine. In late January he earned a rousing welcome from Republicans on Capitol Hill, one suspects more for his disheveled Mr. Brexit stage persona than for his dogged pro-Ukraine activism.

At an Atlantic Council event, Johnson lamented the shrinking spirit of American conservatives. “I’ve been amazed and horrified by how many people are frightened of a guy called Tucker Carlson,” Johnson said. One deadly effective provocateur can spot another.

Rasmussen has made three trips to Washington since last fall, using each to press the argument for Ukraine and promote a plan for Western security guarantees. He says he has met with a number of influential Republicans, including Senators Tom Cotton (R-Ark.) and Mitt Romney (R-Utah), and Rep. Mike McCaul (R-Texas), the chair of the House Committee on Foreign Affairs — all avowed supporters of the fight against Russia.

Actual Ukraine skeptics have been more elusive. I ask Rasmussen if he met war critics like Senator Josh Hawley, the Missouri Republican who has called the war a distraction from a larger struggle with China, or Senator J.D. Vance of Ohio, a new arrival in Washington who as a candidate last year professed indifference to the Ukrainian cause. The answer is a rueful no.

Rasmussen tells me that before visiting Washington he made a list of lawmakers who had criticized the war and requested meetings with some of them. He was ready to tell them that ensuring Russia’s failure in Ukraine was not a distraction from the contest with China, but rather a crucial opportunity for the West to show its collective power and resolve. He wanted to explain how European governments are doing their part against the Russian menace and to stress that “isolationism has never, ever served the interests of the United States.”

Not one war critic agreed to meet him, he says. Unfortunately for me, Rasmussen declines to “name and shame” the specific members who blew him off.

Rasmussen says he tries to go on Fox whenever he’s in the United States, though he has not managed to get on air in months. When I ask if he tried for a slot on Carlson’s show, he chuckles: “Not Tucker Carlson.”

Even a war effort can ask too much.

If American voters’ enthusiasm for the war may not continue forever, then Rasmussen believes we must make it count now. That means giving Ukraine fighter jets, longer-range missiles and other arms Biden has resisted sending. If Trump’s opponents cannot beat him in the primary, Rasmussen hopes that perhaps Ukraine can defeat Russia first. He predicts Biden will wind up sending warplanes, calling it merely “a question of time.”

The former NATO boss has nothing critical to say about Biden. When I suggest the president might do more to explain the war to American voters and address their skepticism, he shrugs off the idea. Biden is the best thing the transatlantic alliance has going.

“We are blessed,” Rasmussen says, “by having a true internationalist and globalist in the White House.”