5 things to watch in New Hampshire's primary

Former South Carolina Gov. Nikki Haley's presidential campaign faces a major test of electability in New Hampshire's Republican primary on Tuesday, when, polls show, she has her best chance to put a dent in former President Donald Trump's chances of clinching a third straight presidential nomination.

Haley is leaning on turnout from independents who can vote in the primary to boost her to a win or a close second to Trump, which, she has said, would fuel her momentum until next month's primary in her home state of South Carolina. But a more expansive victory by Trump could pump the brakes on Haley's argument that she is a viable alternative.

While Haley has closed the gap with Trump in New Hampshire, somewhat, 538's polling average shows Trump up by more than 30 points in South Carolina.

On the other side, Joe Biden's allies are working to gin up support for a write-in campaign for the president, who is not appearing on the ballot in the state's unsanctioned Democratic primary. Despite the odd circumstances of the race, a win against Biden for either Minnesota Rep. Dean Phillips or author Marianne Williamson would mark an embarrassment for the incumbent -- a scenario many leading Democrats in New Hampshire are hoping the write-in effort will avoid.

Here are 5 things to watch in Tuesday's races.

Can Haley really pull off an upset?

Haley's campaign and allies have been bullish that she's set for a win or narrow second-place showing. But a bad loss, following a weak showing in Iowa's caucuses last week, could spell the beginning of the end for the South Carolinian.

Challenger Ron DeSantis, who did slightly better than Haley in Iowa, ended his own campaign on Sunday after he said he didn't see a chance of success in the coming races.

With 538's polling average still showing Haley more than 14 points behind Trump as of Tuesday, her defeat appears likely, according to some Republican strategists.

"Judging by what I'm seeing, I think it's going to be extremely difficult for Nikki Haley to hold Donald Trump to under 50%. And I think there's a chance that Trump pushes 60% tomorrow," New Hampshire's Mike Dennehy told ABC News Monday.

Haley has sounded confident on the trail, especially after DeSantis left the race.

"It's now one fella and one lady left," she said Sunday after the Florida governor made his announcement. "May the best woman win."

But the narrowing of the field, now essentially down to Haley and Trump, could actually be helping him more than her: 538's polling average in New Hampshire shows him recently gaining more in support than she has, perhaps from voters who had backed former challengers like DeSantis or businessman Vivek Ramaswamy.

Trump told Newsmax on Sunday, "Maybe she'll drop out on Tuesday. Let's see what happens."

How many people -- and who -- turn out?

For Haley to perform the way she needs to, she'll likely need strong turnout and support from independent voters, who under New Hampshire law can vote in the GOP primary.

With polls showing Trump romping among registered Republicans, undeclared voters are seen as marking Haley's best path to earning enough votes to remain competitive beyond New Hampshire.

However, the experts who spoke with ABC News were unsure that enough independents will come out to give Haley the support she needs given Trump's expansive margins with Republicans.

"It could happen," said Andrew Smith, director of the University of New Hampshire's survey center, "but she would need to have that be 55% undeclared, 45% registered Republicans [in the turnout]. The highest it's ever been has been 40% undeclared in 2012. And she'd have to get 65% of the undeclareds."

It was originally thought that Haley could be aided by Biden choosing to not compete in the unsanctioned Democratic primary -- but now, a concerted write-in effort could persuade independents eager to back or buck the president to vote in that primary instead.

What's next?

Tuesday night's results could not only dictate the next steps for Haley's campaign but also for the entire 2024 election cycle.

With Haley so far behind in South Carolina polling already, a major loss could undercut her argument about what's next.

"The goal is we wanted to be strong ... stronger in New Hampshire and then even stronger than that in South Carolina," Haley told ABC News' Rachel Scott on Monday, referring to her home state's Feb. 24 primary. "We have saved our money. We've got it ready. We've got the big ad buy that we're going to do for South Carolina and we're going to crisscross the state that I love so much so we're not we're not backing out anytime soon."

Yet if Haley is rejected by voters in the primary race and ends her campaign in the coming days or weeks, that would essentially confirm a Biden-Trump rematch and set off one of the longest general elections in U.S. history.

Biden's campaign is already preparing for that -- and Trump allies have said the same. Biden launched a full court press on abortion rights this week and has been highlighting improvements in the economy and his argument that Trump is a danger to democracy.

Trump, for his part, has been targeting Biden over his age and stamina and negative perceptions on some key issues, particularly the president's handling of inflation and immigration.

Sununu's swan song

Haley's campaign, besides helping determine the next GOP presidential nominee, also marks the swan song for New Hampshire Gov. Chris Sununu, one of her top surrogates who announced last year that he won't run for reelection in 2024.

Sununu has taken a different tact than most surrogates, who appear at some rallies and do some media appearances, opting instead to appear at virtually every rally and absolutely blitz the media. And Haley's showing could mark one of his final shows of political strength -- or lack thereof -- before leaving office.

Sununu is well-liked in his state and he was rumored to be considering both presidential and Senate campaigns in recent years before deciding to stick to his current gig.

He has said he plans on returning to the private sector but has suggested he'll also find a way to stay in the public eye, including continuing the media appearances he appears to relish.

"I like this media stuff," he said in an interview with The New York Times. "I have my quiet criticisms on the media, and I've gotten to see how they do it, what they do, how they work. And some things I'm impressed by. And some things I'm not. So, I thought, 'Oh, maybe I could add a little bit of color to what the media is currently doing and maybe enhance that game.'"

How does Biden fare?

There's notably less action on the Democratic side, but New Hampshire still holds significance for Biden.

The state, led by Republicans, bucked the Democratic National Committee when the party changed up its nominating schedule to make South Carolina's primary the first on the calendar over New Hampshire.

Because the state didn't comply, the DNC labeled Tuesday's primary as pointless and stripped it of any delegates who help award the nomination. Biden, too, declined to appear as a candidate.

Yet Biden's allies are far from abandoning New Hampshire, worrying that a weak victory or a second-place finish to either Phillips or Williamson would mark a blemish on Biden's electoral record before he heads into the general election, likely against Trump.

Neither Phillips nor Williamson have been able to muster much momentum in the polls, but overperforming in the state could breathe new life into their campaigns and elevate the argument that Biden is too disliked to run again.

A strong victory there, though, especially via an unusual write-in campaign, would likely weaken the president's critics.