This week, Republican primary candidates for president will have a chance to make their case before a national audience — with or without Donald Trump on the debate stage. To win, they must break free of Mr. Trump’s drama, step out of his shadow, go on offense, attack, and present their case. Then they need to see if they can catch fire this fall — and if they can’t, they need to step aside, because winnowing down the field of candidates is the single best chance to stop Mr. Trump. Too much is at stake for us to have wishful candidacies. While the other Republican candidates are running to save America, Mr. Trump is running to save himself.

Candidates on the debate stage should not be afraid to attack Donald Trump. While it’s true that Mr. Trump has an iron grip on more than 30 percent of the electorate, the other 60 percent or so is open to moving forward with a new nominee. Mr. Trump’s shortcomings hardly need reciting. Tim Scott, Ron DeSantis, and Vivek Ramaswamy — candidates with compelling stories, records and polling — must show voters they are willing to take on Mr. Trump, show a spark, and not just defend him in absentia. Chris Christie, who has done great work exposing Mr. Trump’s weaknesses, must broaden his message and show voters that he is more than the anti-Trump candidate.

If Mr. Trump is the Republican nominee for president in 2024, Republicans will lose up and down the ballot. According to a recent Associated Press-NORC Center for Public Affairs Research poll, nearly two-thirds of Americans say they would likely not support Mr. Trump in 2024 — not even Jimmy Carter had re-election numbers that bleak. Every candidate with an (R) next to their name, from school board to the statehouse, will be left to answer for the electoral albatross at the top of the ticket. Instead of going on offense and offering an alternative to Joe Biden’s failing leadership, Republicans will continue to be consumed with responding to Mr. Trump’s constant grievances and lies, turning off every independent suburban voter in America. And Mr. Trump, ever the narcissist, will spend the entire campaign whining about his legal troubles and bilking his supporters of their retirement savings to pay for his lawyers.

Donald Trump is beatable, and it starts in Iowa and New Hampshire. Ignore the national polls that show he is leading — they are meaningless. It’s a reflection of the national conversation, name ID, and who is top of mind — not where the momentum is headed.

The best indicator of Mr. Trump’s strength is looking to where the voters are paying attention: in states where candidates are campaigning, television ads are running, and there is a wide range of media attention on every candidate.

In Iowa and New Hampshire, the first two states that will vote in the 2024 Republican primaries, Mr. Trump is struggling. In both Iowa and New Hampshire, he is consistently polling in the low 40 percent range. The floor of his support may be high, but his ceiling is low.

In New Hampshire, more than half of Republican primary voters — our party’s most ardent voters — want someone not named Trump. While he regularly polls above 50 percent nationally, and even closer to 60 percent many times, he has not hit over 50 percent in New Hampshire in the last five months, according to Real Clear Politics.

Having won four statewide elections in New Hampshire and earning more votes in 2020 than any candidate in history (outpacing Mr. Trump’s loss by 20 percentage points that year), I know that in New Hampshire, you don’t only win on policy: You win face-to-face, person-to-person. Voters have to look you in the eye and sign off on you as a person before they can sign off on you as a candidate. Prepared remarks behind a podium do not work.

Candidates who have gone on to win the New Hampshire primary, best illustrated by former Senator John McCain, become omnipresent in my state. You must listen first, talk second. Talking at voters in New Hampshire does not work.

This is why Mr. Trump must face a smaller field. It is only then that his path to victory shrinks. Leaders within the Republican Party — governors, senators, donors and media influencers — have an obligation to help narrow the field.

At a minimum, any candidate who does not make the stage for the first two debates must drop out.

Anyone who is polling in the low single digits by Christmas must acknowledge that their efforts have fallen short.

After the results from Iowa come in, it is paramount that the field must shrink, before the New Hampshire primary, to the top three or four.

Candidates who have essentially been running for years, and who have seen little movement in the polls especially in the early states, are particularly in focus. This fall, if their numbers have not improved, tough conversations between donors and their candidates need to happen. Media influencers and leading voices should amplify the Republican message that the longer these candidates stay in the race, the more they are helping Joe Biden — and Kamala Harris — get four more years.

Provided the field shrinks by Iowa and New Hampshire, Mr. Trump loses. He will always have his die-hard base, but the majority is up for grabs. Candidates who seize on the opportunity and present a clear contrast to the former president will earn the votes.

Candidates cannot continue to let the former president dominate the media like he has for the last six months. They need to be more aggressive about seizing the opportunity to boost their national profiles. There has been positive movement from some candidates, but more needs to be done.

It must be said that candidates who stay in this race when they have no viable path should be called out. They are auditioning for a Trump presidency cabinet that will simply never happen. And even if a Trump administration magically materialized, no public humiliation that great is worth the sacrifice.

As governor of the first-in-the-nation primary state, I will do everything I can to help narrow the field. I plan to endorse and campaign for the best alternative to Mr. Trump. As of now, it’s anyone’s for the taking.

For 20 years straight, the winner of the New Hampshire Republican presidential primary has gone on to secure the party’s nomination. Once the voters of Iowa and New Hampshire are presented a clear alternative to Mr. Trump, his path forward darkens, and the Republican Party’s future begins to take shape. The rest of the country needs to see not just that the emperor has no clothes, but that the Republican Party is able to refocus the conversation where it needs to be, on a nominee dedicated to saving America.