Opinion | How Haley, DeSantis and Other Republicans Could Beat Trump

In the movie “Back to the Future II,” Michael J. Fox’s character, Marty McFly, is transported to the fall of 2015 and encounters a world of self-tying shoes and hoverboards. He finds himself trying to make sense of how people behave and the choices they make.

Lately, I too feel like I’ve been transported to autumn 2015.

That fall, Republican Party officials, donors and operatives were brimming with hope that the field of presidential contenders facing Donald Trump would shrink, clearing the way for a one-on-one matchup between the then-unthinkable Mr. Trump and a more conventional nominee, like the senators Ted Cruz of Texas or Marco Rubio of Florida. As one Michigan donor put it, “Just like everyone else, I’m waiting for this field to consolidate, and it doesn’t seem to be consolidating.” Arguments ensued over which candidates should take the hint. If only the field were smaller, the thinking went, surely Mr. Trump would be defeated.

If only, if only, if only. This line of thinking became an excuse for candidates who didn’t want to take any real swings at Mr. Trump. It was the field — not Mr. Trump — that was the problem. No top contender took him on aggressively. No one really hammered away at his weaknesses in the kind of sustained, nimble assault they would have needed to topple him from his position as the front-runner. And even as Mr. Trump’s opponents dropped out, some of their voters wound up drifting to Mr. Trump as a second choice, keeping him at the head of the pack.

Something similar is happening today. From the Republican debates to the campaign trail, the other candidates aren’t making a real attempt to dent the front-runner’s lead; instead, they are sniping at one another, pleading with donors and engaging in magical thinking. While there is some truth that having more candidates helps Mr. Trump win the nomination, consolidating the field alone is not likely sufficient to defeat him.

In Iowa, late October polling from NBC News, The Des Moines Register and Mediacom showed Mr. Trump at 43 percent among likely G.O.P. caucusgoers, and an analysis of those voters’ second choice shows that a sizable number of Ron DeSantis’s voters would simply go to Mr. Trump were the Florida governor to withdraw from the race. And though the former South Carolina governor Nikki Haley has seen her fortunes improve dramatically since her first debate, she still trails Mr. Trump by a wide margin in her home state of South Carolina, according to the latest CNN polling.

Mr. DeSantis is hoping for some fresh momentum in Iowa this week with the endorsement of the state’s governor, Kim Reynolds. But without bold new arguments from Mr. DeSantis, Ms. Haley or other rivals, it seems as though Donald Trump is increasingly inevitable as the Republican nominee. A shame, too; there is a sizable portion of the Republican electorate that likes Mr. Trump but claims to be open to a new direction. In early states like Iowa and New Hampshire, CBS News polling recently found that fewer than a quarter of primary voters are “Trump only” voters. A vast majority are “Trump and …” voters — people who are considering the former president but also alternatives.

However, these voters aren’t hearing anyone clearly lay out the case for why a new direction is so critical — and they need to on Wednesday night in the third G.O.P. primary debate.

To defeat Mr. Trump, something significant must change. Toughness is required. The math is clear. Mr. Trump’s opponents must take some of his voters from him. Republican candidates must make the case that Mr. Trump is also not the best that voters can do. That means directing at least as much fire at the front-runner as they do at their other adversaries. Mr. Trump’s strongest rivals have not compellingly answered this question: “If you support Donald Trump’s policies so much, why are you running against him?” It’s time they start giving an answer to voters.

Candidates have avoided doing so because striking the right tone in this campaign is incredibly challenging. An attack that is too scathing can inspire a sort of antibody response in Republican voters, backfiring by activating the instinct to defend Mr. Trump. Chris Christie was booed at the Florida Freedom Summit last weekend and hasn’t gained traction in the polls because Republican voters are not looking for a candidate whose primary message is that they have been duped and that Donald Trump is a bad man.

At the same time, the rest of the field has treated Mr. Trump far too gently. Speeches like one Mr. DeSantis gave on Saturday, which try to draw a contrast with Mr. Trump without saying his name, haven’t moved the needle. Vague platitudes about “fresh leadership” and “someone who can win” have not yet been paired with an effective explanation of why Mr. Trump is neither of those things, and as a result, Republican voters don’t believe either is true.

To beat Mr. Trump, the strategy can’t be weak or astringent. It should not scold, and certainly not demean the majority of Republican voters who do like Mr. Trump. It should take a tone more akin to “I’m not mad, I’m just disappointed” — and lay out why, in a sustained and convincing way every week from now through the Jan. 15 Iowa caucuses.

Critically, it must echo and amplify the things that I’ve heard Trump voters who are still shopping around say to each other:

But his mouth, they’ll say.

“I don’t like when he makes things like a circus.”

“It’s just tiring.”

These are not attacks from the right flank on policy, nor attacks from centrists on his legal woes — these are truths that many of Mr. Trump’s own supporters will acknowledge in private conversation.

Mr. Trump’s opponents should not shy away from pointing out how things were far from perfect during his presidency. As one Trump voter acknowledged in one of our recent Times Opinion focus groups, “He always said everything is wonderful and everything is beautiful and everything is amazing. Come on.”

In Wednesday’s debate, even with Mr. Trump absent from the stage, he is looming over his rivals, and it is past the hour for them to act like it. They can try to raise more money, but they cannot raise more time.

Take Ms. Haley. She can, for instance, make a strong case for her actions defending Israel during her tenure as U.N. ambassador, noting that while the world may have felt safer under Mr. Trump, his own personal drama with Prime Minister Benjamin Netanyahu has blinded him to the importance of supporting one of our most critical allies — and that this kind of self-interested behavior will keep Mr. Trump from doing what is necessary to support our friends and go after our enemies on the world stage in a second term. Based on new polling from The Times and Siena College, she can now also make a clear electability case that has thus far eluded Mr. DeSantis, and note that she is the safer, stronger bet against Mr. Biden in 2024.

Taking Mr. Trump on directly is no easy task in a party that would vastly prefer him to be in the White House today. But avoiding the hard road has thus far put his adversaries on the path to defeat. At this stage of the primary season, with the lead Mr. Trump holds, there is no longer an excuse for candidates to dance around the question of “Why you … and not Donald Trump?”