US Republican presidential candidates, including Donald Trump, rallied for support just ahead of Monday night's Iowa caucus, the first of the primaries for the GOP race.

Trump is leading the pack, which includes former UN Ambassador Nikki Haley and Florida Governor Ron DeSantis. They each held rallies on Sunday to appeal to voters as frigid cold gripped the Midwestern state.

Opinion polls show Trump has over 60 percent support on average among Republican voters.

The former president assured his supporters that they would "evict crooked Joe Biden from the White House" by winning the presidential election in November. He pledged never to leave the people behind.

Haley lashed out at Trump, saying "rightly or wrongly, chaos follows him."

She said, "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."

DeSantis said Trump is running "to pursue his issues" and Haley "for the donors' issues."

He said he is the only one running with a focus "solely on your issues and your family's issues, turning this country around."

The Iowa caucus is traditionally regarded as a bellwether for who would prevail in the race. But the record cold, with temperatures dropping below minus 20 degrees Celsius, could affect voter turnout and impact the results.

The Republican primaries are kicking off with Trump, the frontrunner, facing four criminal trials.

In one, Trump has been indicted on charges of trying to overturn the results of the 2020 presidential election, which led to the attack on the US Capitol in January 2021.

The trial on the case is scheduled to begin on March 4, one day before Super Tuesday, when primaries and caucuses will be held in many states.

Another indictment accuses Trump of falsifying business documents to conceal hush money paid to an actress. The trial opens on March 25.

Trump also faces charges of illegally retaining documents, including highly classified information on military capabilities, at his home after leaving office. The trial of this case is scheduled to begin on May 20.

There is also doubt about Trump's eligibility to run for president. The top court in the western state of Colorado has deemed him ineligible for the state primary, saying he was involved in insurrection by inciting his supporters to attack the US Capitol in 2021.

Trump has appealed this decision to the US Supreme Court. Oral arguments are scheduled on February 8.

The New York Times says similar legal challenges over Trump's eligibility have been filed in 35 US states, of which decisions are pending in 17.