Candidates sometimes run for president to become vice president.

Frequently, presidential candidacies are more tryouts for vice president than they are for the top job.

And sometimes a failed but lucky presidential candidate gets picked for the second spot.

It happened to Kamala Harris in 2020, and it could happen to Nikki Haley in 2024.

President Biden ended up with then-Sen. Harris, a Democrat presidential primary opponent in 2020, even after a contentious clash over race during a presidential debate.

And former President Donald Trump could end up with Haley, his former ambassador to the United Nations, as his vice-presidential running mate even after Trump called her, among other things, a “birdbrain.”

Trump shamelessly calls a lot of people a lot of things.

While neither Joe Biden nor Kamala Harris called each other “birdbrains” during the 2020 primary campaign, they did argue over race during a debate.

And although Harris did not call Biden a “racist” — as it was later wrongfully reported — she did say it was “hurtful” to listen to Biden talk about his favorable relationship with two former segregationist senators as well as his opposition to school busing.

No matter their differences, Biden took Harris — a U.S. Senator and a woman a of color — as his running mate and they went on to defeat Trump and Vice President Mike Pence in the close 2020 election.

Now Biden and Harris are running again, and both are woefully underwater in all the public opinion polls.

Despite all of Trump’s legal problems — and the growing sentiment that the Biden Justice Department and FBI will do anything to destroy him — the former president is ahead of Biden in just about all the polls.

But so isn’t Haley.  While a Wall Street Journal poll of 1,500 registered voters last weekend showed Trump defeating Biden by 4 points, the same poll showed Haley beating Biden by a whopping 17 points.

While the numbers are great for Haley, she and the other GOP candidates trail Trump badly among Republicans, including in Iowa, the first caucus state.

No matter the poll, Trump remains the presumptive favorite to be the Republican nominee for president in 2024.

Haley knows that. She (along with Trump) also knows that she would be a tremendous asset to Trump on the campaign trail.

Can the two get along? Who knows?

Trump said, “she criticizes me one minute, and 15 minutes later she uncriticizes me,”

Haley has shown she is a smart, attractive, and stand-up campaigner able to debate and mix it up with GOP candidates Ron DeSantis and Vivek Ramaswamy. She would do as well or better with Kamala Harris

Asked in an ABC-TV interview Monday about Trump’s “fitness” to be president again, Haley said, “It’s not about fitness. I think he’s fit to be president. It’s ‘should he be president?’ I don’t think he should be president. I thought he was the right president at the right time.

“We’ve got to look at the issue that we’re dealing with, coming forward with new solutions, not focusing on negativity and baggage of the past. So, it’s not about being fit. I don’t think he’s the right person to be president,” she said.

Asked on another occasion about her waffling loyalty to Trump, who made her an ambassador, Haley said, “I’ don’t agree with Trump one hundred percent of the time. I don’t disagree with him one hundred percent of the time. That’s (also) the way I feel about my husband.”

Haley’s husband is Major Michael Haley of the South Carolina National Guard, who is currently deployed in Africa following a previous deployment in Afghanistan.

Just because politicians in the heat of a battle call each other names does not mean they cannot unite when it serves their purpose, or turn a negative into a positive.

When Trump, on his media platform Truth Social, singled out Haley and called her a “birdbrain” following the second GOP debate, Haley responded by saying, “Love this. It means we are second and moving up fast.”

And she has.

Peter Lucas is a veteran Massachusetts political reporter and columnist.