Her polling numbers, donations and endorsements have surged in recent weeks.
But her long-nurtured hope of a one-on-one battle with the ex-president was dealt a blow when Florida governor Ron DeSantis beat her to the runner-up spot in Iowa's opening nominating contest on Monday.
The disappointment ratcheted up the pressure on Haley to put DeSantis in the rearview mirror in New Hampshire on Tuesday, while keeping alive her challenge to Trump as the race moves to her home state of South Carolina.
"If Haley has a good showing in New Hampshire, it's going to shift the momentum, energy and excitement behind her campaign going into South Carolina," said Haley backer and former Maryland governor Larry Hogan.
- Ball game -
No candidate has ever lost the race after claiming the first two states, and Trump, who crushed DeSantis and Haley in Iowa, would almost certainly declare the Republican nomination over with a win in New Hampshire.
That has made the Granite State the whole ball game for Haley, with fundraisers reportedly threatening to jump ship if she falls short of a win or a very close second to Trump.
Speaking Wednesday in the small city of Rochester, Haley told supporters that she voted for Trump twice herself, but that "we can't have a country in disarray, a world on fire and go to four more years of chaos. We won't survive it."
"You don't defeat Democratic chaos with Republican chaos," she said, hitting out at both Trump and President Joe Biden in one fell swoop.
Trump's race appears to get easier in February as more conservative southern states begin to weigh in ahead of the "Super Tuesday" stampede of results from 16 states and territories on March 5.
But he has to get through New Hampshire first, where his support has ebbed as Haley has upped her favorability by spending more time on the ground.
One of the factors that makes the state tougher terrain for the real estate tycoon is the large proportion of independents who vote in primaries and typically break for more moderate candidates.
Haley has pegged back Trump's average polling lead from 35 points in the fall to just 14 -- and she has dismissed DeSantis, who is running a distant third in New Hampshire, to cast the Republican primary as a two-horse race.
Former New Jersey governor Chris Christie's decision to drop out was seen as a boost to Haley, as was an endorsement from New Hampshire's popular Republican governor, Chris Sununu.
But the daughter of Indian immigrants has raised eyebrows on the campaign trail over race -- first for failing to identify slavery as the cause of the Civil War, and then for asserting on Tuesday that America has "never been a racist country."
- 'Dog whistle' -
Trump's supporters tolerate transgressions that would sink Haley or any other candidate, and his attacks on the former South Carolina governor have taken on a distinctly racial bent.
He referred to her by her first name, Nimarata, on Tuesday in what was reported as a racist "dog whistle" drawing attention to her heritage.
And he has promoted a post falsely claiming that the 51-year-old was ineligible to run for president because her parents were not US citizens at the time of her birth.
Haley has walked a tightrope between hitting back at Trump over the many controversies surrounding his candidacy and trying to keep his ultra-loyal supporters onside.
She has downplayed the 91 felony charges he faces, and declined to weigh in on Trump being found liable for sexual abuse, stating that she hadn't "looked at" the case.
But she sharpened her attacks Wednesday as her campaign circulated a memo depicting Trump as "more vulnerable than commonly believed" and promoted television ads depicting Trump as a bully and a liar.
Haley also announced that she would not take part in two debates ahead of the primary unless Trump joined her on stage, reinforcing her message that DeSantis should no longer be seen as a credible candidate.
