Haley is going head-to-head with Trump in New Hampshire. Some experts expect it won't help.

Republican critics of Donald Trump have always said that getting the 2024 primary down to a one-on-one race would be key for any of his challengers to try to defeat him with voters.

That race is now here -- and by Tuesday night, it'll start to become clear whether it matters.

The former president will face off against just former South Carolina Gov. Nikki Haley in New Hampshire's primary on Tuesday after Florida Gov. Ron DeSantis, entrepreneur Vivek Ramaswamy and former New Jersey Gov. Chris Christie bowed out in recent days. (Technically, Trump and Haley are also running against pastor Ryan Binkley, but he has failed to register in polling and never qualified for the debates.)

The head-to-head, combined with New Hampshire allowing independent voters to participate in the Republican race, is seen as offering Haley her best shot at throwing a blockade in front of Trump's campaign before it becomes a runaway train.

If Haley can pull off a strong showing in the state, she has indicated, she can use that momentum to argue in other states that a Trump win is not inevitable and voters can go with someone else.

"We're going home to South Carolina. 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, 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."

But Republican strategists in New Hampshire estimated to ABC News that while Haley is likely vacuuming up former Christie supporters, many DeSantis and Ramaswamy backers are now swarming to Trump, because his policies and approach are more similar to theirs, dampening anti-Trump Republicans' hopes that there is a large enough vote to consolidate against him.

And polling offers a dour outlook for Haley, with only one major survey in 538's database showing her within single digits of Trump -- and none showing her near the lead, though she is running closer to Trump in New Hampshire than elsewhere in the country.

"I think President Trump ends up basically locking up the nomination tomorrow night around nine o'clock," predicted New Hampshire GOP strategist Dave Carney.

"No other state in the country do you have this high of independents in the mix," Carney added. "And if you can't beat Trump here, you can't beat him anywhere. He's going to get over 50%, and there's no pathway for anyone else to move forward, and the race will be over. There's nobody siphoning off Trump voters from Trump now that it's a two-way race. It's going to be a beatdown."

Such a result would mark a significant blow for Haley, who is looking for a win or a strong second-place showing to catapult her to next month's primary in her home state of South Carolina, though that state's soil is even more fertile for a Trump win, according to 538's polling average.

Trump is currently leading Haley in South Carolina by more than 30 points, per 538.

Haley and her campaign surrogates have boasted of her chances in New Hampshire. And while they have moderated expectations more recently, going from predicting an outright win to saying that running narrowly behind Trump would be enough to keep her going, the former governor is still sounding a bullish note on the trail.

"It's now one fella and one lady left," she said at a campaign stop on Sunday after DeSantis suspended his campaign. "May the best woman win."

Republican and nonpartisan operatives in the state said that to win, Haley would need to overperform recent polling on how she fares with undeclared voters -- who make up the plurality of voters in the state. But she'd also need for those voters to turn out at virtually historic levels, given Trump's continued popularity with registered Republicans.

"Undeclareds always vote at lower rates than do registered Republicans, and no candidate's ever won the primary without winning the plurality of the party registered voters. Nobody's ever won with independent voters," said Andrew Smith, director of the University of New Hampshire's survey center, referencing possible turnout.

"It could happen," Smith said, "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."

Carney said that kind of turnout for independents in the GOP primary may be particularly difficult to achieve given competing activity on the Democratic side. President Joe Biden's allies are working to gin up support for a write-in campaign in the party's unsanctioned primary, which is also featuring two long shot challengers, Minnesota Rep. Dean Phillips and author Marianne Williamson.

"Some undeclared voters are gonna say, 'I can't stop Trump, Trump's gonna win, I'm gonna go vote against the president or ... I'm gonna go vote for Biden.' We are historically late deciders, and undeclared voters have the luxury of voting where they can have the most fun, the most impact, cause the most trouble," Carney said. "If Biden was on the ballot and he was getting 92 [or] 82% of the vote, there's no reason to vote in the Democrat primary. I think it would help Haley much more."

Ensuring strong support from undeclared voters is expected to be even more important for Haley after Ramaswamy and DeSantis dropped out, strategists said. The two were seen as largely competing with Trump for votes, given their similar platforms, and their departures could add to his already hefty advantage, some experts said.

"That's why I say I think Trump could push 60[%] because what Haley needed to have happen in this last week -- she needed Vivek to stay in the race and she needed DeSantis to stay in the race to split the Republican vote," said New Hampshire GOP strategist Mike Dennehy. "Both of those candidates dropping out has boosted Donald Trump up 7 to 10 points."

Indeed, 538's polling advantage has seen Haley gain support in recent days -- but Trump gaining even more.

Dennehy also said he felt Haley played it too safe and "sat on the fence too long" in New Hampshire, possibly dampening her support.

While she has been blitzing the state with retail stops -- dozens more than Trump -- Haley refrained from many media interviews and town hall events where reporters and voters could press her on the issues and refused to debate DeSantis while he was still in the race.

She has ramped up her pointed attacks on Trump, though for months she spoke more ambivalently, casting him as the "right" president while in office but also an agent of chaos. She's said she would pardon him if she were elected and he were convicted of a crime (he denies wrongdoing), and she would support him if he is the ultimate GOP presidential nominee.

"When you come in here from Iowa, you have five, six solid days of campaigning [after the caucuses], you need to use every single resource available, every tool in your belt. And it's like she was fighting with two hands tied behind her back," Dennehy said. "You're not going to motivate and influence independent voters by having supporter rallies. ... She's not acting like the insurgent candidate that she is."

Haley's campaign did not respond to a request for comment from ABC News about the race. And not every operative had similar nitpicks with the way she's run the race, arguing that not alienating Trump voters, who are a significant part of the Republican base, is a valid strategy.

But every person who spoke to ABC News for this story pointed to one overarching theme: Strategy aside, Trump seems to remain the most popular politician in the Republican Party, and ousting him in a primary is a tall task, even with independent votes in one state.

"I think she ran an excellent race," Smith said. "I think she played her cards as best as could be played. But they just weren't high enough cards."