In the Super Tuesday primary races in the United States, multiple US media outlets say former US ambassador to the United Nations, Nikki Haley has decided to withdraw from the presidential race.

It comes after Donald Trump swept almost all of the Republican contests, paving the way for a faceoff with President Joe Biden inNovember.

Haley was Trump's last remaining rival for the Republican presidential nomination. She captured Vermont on Super Tuesday but it was just her second win, along with Washington DC.

US media say she is expected to deliver her exit speech shortly. Haley's withdrawal makes it certain Trump will win the party's nomination.

Trump has secured 14 out of 15 states including California and Texas, the two most populous in the country. Speaking to supporters, Trump hailed his victories and attacked Biden.

On the Democratic side, Biden carried all 15 states, losing only in the US Territory of American Samoa. Reacting to the Republican results, Biden said in a statement, "Are we going to keep moving forward or will we allow Donald Trump to drag us backwards into the chaos, division and darkness that defined his term in office?"

Trump is also facing four criminal trials and multiple civil cases that could hinder his campaign. He may have to use election funds to pay legal expenses.

Recent polls suggest some Trump supporters may change their allegiance if Trump is convicted in the criminal cases.

Meanwhile as the oldest president in US history, Biden's physical and mental fitness have been watched closely ever since he took office. Some voters have voiced concerns over his ability to effectively serve another four-year term.

Also on Tuesday, US pop star Taylor Swift took to social media to remind her fans to vote. She urged her more than 280 million followers on Instagram to back the people who most represent them. She did not name a specific candidate.

In the previous election, she expressed support for Biden but has yet to publicly take a clear stance this time.