To Jacob's point, the "go ahead and underestimate me" phrase is a very clearly gendered one. And Trump's reputation on this issue hasn't gotten as much attention as it did in 2016, but it's not clear that some of the issues have gone away. Plus, Haley may also be highlighting the fact that, as far as she is from being a real contender for the nomination, this is the closest any GOP woman has come.

—Julia Azari, 538 contributor

People forget this, but McMaster was the first statewide elected official to endorse Trump back in 2016. Haley’s appointment to U.N. ambassador cleared the way for him to become governor, a post he still holds today.

—Nathaniel Rakich, 538

Nathaniel, I think that another explanation for Haley’s continued presence in the race is less political and more personal. Clearly, the two candidates don’t get along, and never really have. (Remember that the only reason Trump even tapped Haley as ambassador was as a favor to then-Lt. Gov. McMaster.) With the race getting increasingly chippy and personal, I think Haley may be staying in for longer than politically savvy because she knows just how much it gets under Trump’s skin. This is, after all, a politician who sells shirts that say "Go ahead and underestimate me, that’ll be fun."

—Jacob Rubashkin, Inside Elections

It was pretty clear Trump was going to be the nominee after New Hampshire. I think maybe Haley could be forgiven for trying to stay in for a couple more weeks to see if she could move the needle in South Carolina, a state she knows so well. But after it was clear that Trump’s polling lead wasn’t shrinking, she lost any practical justification for staying in the race.

—Nathaniel Rakich, 538

It's not a huge surprise that Haley thinks she can win tonight despite the odds. Her first political success was in a 2004 South Carolina state House Republican primary, in which she unseated a powerful 30-year incumbent to win. She tells that story on the stump, and the message is clear: She's won as an underdog before.

As a legislator, Haley carved out a reputation in South Carolina as an outsider taking on state power. She sponsored a bill to force roll call votes on issues like raising legislator pay, losing support from her colleagues and a race for a committee chair position in the process. Later, as governor, she became known for luring businesses to the state, siding with business over labor, signing a 20-week abortion ban and blocking Medicaid expansion.

On social issues, though, she has tried to walk a middle path. Haley has talked about the racism her Indian American family faced in rural South Carolina, and supporters have said the "good ol' boys" network never quite accepted her in state politics. But she also frames her success as an example of how the South has made progress. She didn't tackle the issue of the Confederate battle flag flying over the State House until after a racist shooting in Charleston in 2015, in which nine churchgoers were killed in Emanuel AME Church. She has also sidestepped more recent controversies, like anti-transgender bathroom laws.

Of course, all of her history in the state hasn't seemed to help her against Trump. In polls, she's winning only about a third of potential voters. She may be used to playing the role of David, but not all Goliaths fall in defeat.

—Monica Potts, 538