After home-state humbling, the question now is: What's Nikki Haley running for?

It's become a recurring joke in recent Nikki Haley campaign speeches where she dismisses various theories about why she's still in the U.S. presidential race.

Questions about her continued participation will only escalate after her lopsided defeat in her own state of South Carolina as Donald Trump appeared headed for a roughly 20-point victory margin over Haley, the state's former governor.

This is after Haley already invited the media earlier in the week to a decidedly unusual announcement for a candidate: She's not dropping out.

Haley has lost the first four states. She even lost the race in Nevada where Trump didn't appear on the ballot, with Republicans there opting for "none of these candidates." She's now lost in her own state of South Carolina by a wide margin, and she's trailing him by dozens of points in national polls.

Does she want to be Trump's running mate? Haley laughs off the idea in recent speeches, given how their relationship has soured. He regularly refers to her as "birdbrain."

Is she setting up a future run? The news outlet Politico speculates as much. Or is she trying to build a brand outside politics?

"I think she's running for a board seat," Republican strategist Terry Sullivan told a podcast titled What's Nikki Haley's Endgame? Or perhaps, he added: "Maybe she's going to be [a TV host] on The View."

There's one scenario Haley doesn't mention in those stump speeches, and it's the one likeliest to emerge in conversation with political insiders.

It's the possibility that she's running to cement her status as a Plan B. As a backup should Trump be forced off the ticket, either by a health setback or by winding up in jail following a conviction in one of his criminal cases.

This scenario will continue to animate conversation after her home-state defeat, as Haley promises to stay in the race at least through Super Tuesday on March 5.

Two-dozen state primaries will have happened by then, half the country. Haley said it's clear from her approximate 40-per-cent score in South Carolina that many Republicans want an alternative to Trump.

"I have a duty to give them that choice," Haley said in a buoyant speech Saturday night. "Today is not the end of our story. We're heading to Michigan tomorrow."

Talking to primary voters, it's clear that Haley supporters want her staying in. Some of these Haley supporters are Democrats. A higher-than-usual number crossing partisan lines to vote in this year's Republican primaries.

"I'm here to vote against Donald Trump," said retired Brooklyn school teacher Diane Spignardo, a Democrat who now lives in South Carolina and voted in the Republican primary outside Charleston on Saturday.