Two Republicans jostling for a distant second place behind Donald Trump in their race to be party flag-bearer took potshots at the former US president on Thursday, days before the first vote in the White House nomination process.

Florida Governor Ron DeSantis and one-time UN ambassador Nikki Haley turned their fire on their party's presumptive nominee just 11 days before Republican voters in Iowa make their choice.

Neither candidate mentioned the other much during back-to-back "town hall" events on CNN.

Instead, they both sought to paint their own candidacies as a better bet than Trump to unseat Democratic President Joe Biden in November's presidential election.

"The reality is, rightly or wrongly, chaos follows (Trump)," Haley, a former South Carolina governor, told an invited audience in the Midwestern state.

"We can't have a country in disarray, and a world on fire, and go through four more years of chaos. We won't survive it."

Haley has increasingly looked like the establishment Republican candidate for a section of the party desperate to leave Trump behind.

She pointed Thursday to polls that show her with a double-digit win over Biden in a general election, which she claimed would give her a mandate to implement Republican policies.

"A mandate to secure our border, no more excuses. A mandate to bring law and order back to our country and a mandate of a strong America that we can be proud of.

"That's what I think we need to do. It is time to move past president Trump and it is time to start focusing on how to strengthen America."

DeSantis, who started 2023 flying high as the most likely non-Trump candidate in a crowded field but has since lost altitude, also sang a similar tune about the need to move on this November.

"You don't want it to be a referendum on Trump and the past," he said.

"You want it to be a referendum on Biden's failures, on our positive vision for this country. I offer that."

DeSantis also called out his one-time ally Trump for failing to campaign properly in Iowa, in contrast to his own efforts that have seen him visit all 99 counties in Iowa.

"Donald Trump's not willing to show up on the debate stage," he said.

"Has he come to communities and answered questions, has he gone to all 99 counties? Heck, has he even gone to nine counties? That's not the way to do it."

Trump, 77, faces 91 felony charges over business fraud, mishandled classified documents and a criminal conspiracy to overturn the 2020 election.

He has parried those charges, calling them a political witch hunt, much to the delight of his vocal right-wing base.

Party rivals have previously pulled their punches, wary of alienating that base, but on Thursday, both candidates went for him.

In a bold strike sure to ruffle feathers in the largely anti-abortion Republican Party, DeSantis blasted Trump on the issue.

"Of course not," he said when asked if Trump was pro-life.

"When you're saying that pro-life protections are a terrible thing, by definition, you are not pro-life."

Haley addressed Trump's legal woes, hinting she would pardon him if elected.

But, she noted darkly: "Nobody gets pardoned if you're not guilty."

Despite assured performances from each of them on Thursday, polls suggest both Haley and DeSantis have a mountain to climb.

The latest edition of the influential Des Moines Register Iowa Poll showed 51 percent of likely Republican caucus-goers picking Trump, up from 43 percent in October.