Democrat Scott Colom holds a narrow lead over Republican Senator Cindy Hyde-Smith in Mississippi's US Senate race, three days after voters went to the polls, but the contest remains uncalled with thousands of ballots still to be counted.

Colom led by about 2,000 votes, 49.1% to 48.8%, with 96% of votes counted as of midday on Friday, according to the Associated Press. Later unofficial figures put his lead at 2,498 votes, with Colom on 467,846 votes to Hyde-Smith's 465,348.

The outcome will decide how large a Democratic majority the party holds in the Senate after it won control of Congress in this week's midterm elections, in a result widely seen as a repudiation of President Donald Trump's second term.

Ballots still to count

Mississippi law allows mail-in ballots postmarked by Election Day to arrive for up to five business days afterwards, with Tuesday, 10 November the last day they can be counted. County clerks then have until 13 November to certify their results.

About 20,000 ballots were estimated to still be outstanding, according to Mississippi Public Broadcasting, including mail-in and affidavit ballots. Voters who cast affidavit ballots because they lacked photo identification have until 10 November to present ID.

Counties had reviewed 11,202 affidavit ballots by Friday afternoon, accepting 8,717 and rejecting 2,485. Hyde-Smith said she was not backing down, writing: "We are not backing down. This election is not over."

A race that could make history

If Colom's lead holds, he would be the first Democrat to win a US Senate race in Mississippi since John Stennis in 1982, and the first Black Democratic senator from the state.

The race drew national attention after a Colom campaign advert went viral, mocking Hyde-Smith's suggestion that people worried about beef prices could "pick another protein".