The head-to-head showdown comes five days before the pivotal first nominating contest in Iowa, considered crucial for winnowing the field and giving those left standing a springboard for the rest of the contest.
Trump has a commanding lead despite the multiple legal challenges he faces, but has skipped the televised debates, concluding that he has nothing to gain by taking prime-time hits from lower-polling rivals.
None of the other challengers met the qualification standards, leaving the stage for Haley and DeSantis to vie one-on-one for the Hawkeye State's undecided voters.
Haley hit out at Trump ahead of the event at Drake University, in Iowa capital Des Moines, for declining to face his opponents.
"With only three candidates qualifying, it's time for Donald Trump to show up. As the debate stage continues to shrink, it's getting harder for Donald Trump to hide," she said in a statement.
The ex-president will be encouraged by new Suffolk University/USA TODAY polling showing 51 percent of Republicans don't plan to watch, reflecting the low stakes that many voters see in the "undercard" contest.
According to the RealClearPolitics polling average, Trump leads the field in Iowa at 52.3 percent, with Haley and DeSantis in a margin-of-error tussle at 16.3 and 16 percent respectively.
The national picture is similar, although Trump's lead is even larger -- a daunting 51.5 percentage points.
Much of the campaign activity in Iowa has been overshadowed in the final week by the legal woes facing Trump, who has sought to use the precincts of courthouses across the country to dominate TV coverage and rally support.
He ducked out of campaigning on Tuesday for a hearing in Washington, where he faces charges over an alleged conspiracy to overturn the 2020 election, and he is due back in court Thursday for his civil fraud case in New York.
DeSantis, the governor of Florida, was praised for his strongest performance in the fourth debate in Alabama last month, but his campaign has never lived up to the initial hype.
The conservative hard-liner is seeking to regain ground lost to Haley and his path to the nomination depends on a strong showing in Iowa.
DeSantis has also criticized Trump for refusing to participate in the debates, telling reporters the ex-president is dodging his duty to answer questions over his policies and record.
"He parachutes in for (a) 30, 45-minute, hour speech and then just leaves, rather than listening to Iowans, answering questions and doing, I think, what it takes to win," he said at a recent event in Elkader, Iowa.
Haley is looking to outperform expectations in Iowa and ride into a one-on-one match-up with Trump in her preferred state, New Hampshire, where independents are allowed to vote and could give her campaign a significant boost.
She has had a few recent missteps on the campaign trail, failing to identify slavery as the cause of the Civil War and facing criticism for her suggestion that New Hampshire voters would "correct" whatever happens in Iowa.
But the stumbles appear not to have arrested her late surge in polling, endorsements and fundraising, prompting Trump and his allies -- who ignored her for much of 2023 -- to ramp up attacks.
Trump, who often arranges "counterprogramming" to draw attention away from the debates, announced that he will take part in a Fox News town hall event elsewhere in Des Moines as DeSantis and Haley are onstage.
CNN is due to host another debate on January 21 in New Hampshire, two days ahead of the Granite State's nominating contest.
