Donald Trump won 14 primary elections in the "Super Tuesday" states, including Texas and California, but lost out on a clean sweep after Nikki Haley's surprise upset in Vermont.

The former president, bidding for a sensational White House comeback after being unseated by Democrat Joe Biden in 2020, took home an almost-decisive haul of delegates on Tuesday in his march to the Republican Party nomination.

Alabama, Alaska, Arkansas, Colorado, California, Maine, Massachusetts, Minnesota, North Carolina, Oklahoma, Tennessee, Texas, Utah and Virginia were all called by US networks for Trump, who expressed thanks on his Truth Social site.

His longshot challenger, former UN ambassador Haley, narrowly won the northeastern state of Vermont. She has so far refused to drop out of the race.

In the Democratic nomination contest, as expected, Biden won all 15 states, though he lost to little-known challenger Jason Palmer in the small Pacific Ocean territory of American Samoa.

Polling averages from RealClearPolitics show Trump two points ahead of Biden in a one-on-one match-up in the November election.