Many Republicans voting in U.S. presidential primary elections on Super Tuesday said immigration was their top issue and that immigrants in the country illegally should be deported, a campaign pledge made by Donald Trump.

The former president aimed to vanquish his sole remaining challenger for the Republican nomination, Nikki Haley, on Super Tuesday, the biggest day in the presidential nominating calendar when 15 states and one territory were voting.

The first polls were due to close at 7 p.m. in Vermont and Virginia, with voting wrapping up in Alaska at midnight.

Trump has pledged to carry out the largest deportation of immigrants in U.S. history if he defeats Democratic incumbent Joe Biden on November 5.

In three states where voters headed to the polls, 69 percent of Republican voters in California, 62 percent in North Carolina and 59 percent in Virginia backed deporting illegal immigrants, according to exit polls by Edison Research.

In North Carolina, a general election battleground state, a majority of voters polled – 49 percent – favored a national abortion ban, compared to 45 percent who oppose one. But a majority of Republican voters in California and Virginia did not favor a ban.

Abortion is an issue that has caused the party problems since a conservative-leaning Supreme Court ended a federal right to the procedure.

Overall, immigration and the economy were cited by Republican voters in the three states as their top concerns, with immigration the top issue for voters in North Carolina and Virginia.

Trump, who has dominated the Republican campaign from the start despite a litany of criminal charges, has swept all but one of the contests so far, winnowing a sprawling Republican field of candidates down to two.

While Trump cannot win enough delegates to formally clinch the nomination on Tuesday, another dominant performance would further pressure Haley, a former South Carolina governor and his remaining rival.

The day's contests will award more than one-third of Republican delegates – and more than 70 percent of the number needed to secure the nomination.

Biden was expected to win Tuesday's Democratic contests easily, though activists opposed to his strong support of Israel were calling on Muslim Americans and progressives to vote "uncommitted" in Minnesota in protest.

Haley, a former United Nations ambassador under Trump, has faced mounting questions about how long she will continue her long-shot campaign, particularly after losing her home state of South Carolina 10 days ago.

Trump told Fox that his focus was on Biden, adding: "We're going to win every state tonight."

Biden said in an interview on Power 98 FM, a hip-hop and R&B radio station that serves Charlotte, North Carolina, that the elections were a chance to take on "the extreme division and violence the MAGA Republicans are pushing," using the acronym for Trump's Make America Great Again campaign slogan.