With a month to go before Election Day, the presidential race is effectively tied. Neither Vice President Kamala Harris nor former president Donald Trump has opened a convincing lead outside the margin of error in any of the key battleground states, according to the RealClearPolitics average.

There are plenty of persuadable voters in those states. According to the most recent New York Times-Siena College poll, about 18 percent of likely voters in several key swing states say they haven’t yet made a firm decision. (Some are undecided, while others say they are leaning toward Trump or Harris but could still change their minds.)

An overwhelming 43 percent say it is Trump’s temperament and trustworthiness — his character — that make them reluctant to put him back in the Oval Office. They might feel that their lives were better under Trump. They might even like his polices. They just don’t like him. They don’t like the things he says. They don’t like the way he acts. But they also have deep concerns about Harris, including her temperament, ideology, honesty and experience level. And so they are struggling with their vote.

What if there was someone who shares their concerns about Trump but is supporting him anyway — and might be able to help persuade them to do the same?

There is. Her name is Nikki Haley. The former U.N. ambassador is the very model of a reluctant Trump voter who has put aside her reservations and decided to pull the lever for the former president.

So, why is Trump not deploying her on the campaign trail?

Haley’s message of strength unity is precisely the message swing voters need to hear every day between now and Election Day. But, since the convention, Trump has not asked Haley to join him on the campaign trail. Last month, Haley reiterated her willingness to help. On CBS’s “Face the Nation,” she was asked if she planned to campaign for the former president. “He knows I’m on standby,” Haley said. “I talked to him back in June and he’s aware that I’m ready if he ever needs me to do that … but there hasn’t been an ask as of yet.”

So, ask! I get that Trump considers her disloyal for having run against him. But who cares? Like it or not, Haley got over 4,415,000 votes in the Republican primaries this year — including a whopping 972,343 in the key battleground states that will decide the next president (110,966 in Arizona; 77,902 in Georgia; 297,124 in Michigan; 250,838 in North Carolina; 158,672 in Pennsylvania; and 76,841 in Wisconsin). Trump lost four years ago by just 42,918 votes in three battleground states. In other words, her supporters could very well determine whether he wins or loses this election.

Haley voters are precisely the swing voters Trump should be targeting — Americans who are willing to vote for a Republican but are hesitant to do so for Trump. He should be appearing with her at rallies and basking in her endorsement. He should deploy her to key battleground states for the next four weeks to convince the reluctant but persuadable 18 percent of the electorate to cast their ballots for him. Failure to do so would be sheer political malpractice.

It could very well cost him the presidency.

Follow Marc A. Thiessen @marcthiessen on X.