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