President Donald Trump held two rallies for a controversial Republican US Senate candidate from Mississippi ahead of a run-off vote in the southern state on Tuesday in an election hounded by concerns over race.

Trump campaigned for Cindy Hyde-Smith on Monday, the last day of campaigning, as the Republican party poured money into the run-off vote.

Hyde-Smith has drawn national attention over her remarks with deep racial undertones, including when she said about a supporter: "If he invited me to a public hanging, I'd be on the front row."

Republicans have thrown their financial and political weight behind the embattled candidate.

A photo from 2014 on social media also emerged, showing her holding a rifle and wearing a cap of a soldier from the southern Confederacy, the rebel movement that battled the federal government in the US civil war of the 19th century, in part to keep slavery alive.

She is running against Democrat Mike Espy, an African-American who served in Washington under the administration of President Bill Clinton.

"Your vote on Tuesday will decide whether we build on our extraordinary achievements or whether we empower the radical democrats to obstruct our progress, reverse our gains and return to the failures of the past," Trump said during a rally in Tupelo Mississippi on Monday.

"She votes for us and she votes for 'Make America Great Again,'" Trump said of Hyde-Smith.

Taking questions from reporters in Mississippi, Trump said that he "heard an apology loud and clear" from Hyde-Smith regarding her "public hanging" comment.

"Her heart is good. That’s not what she was meaning when she said that," Trump added.

Corporate donors, including the baseball league, have asked to claw back donations to the candidate.

What was considered a sure bet for the Republicans has become a close race, though Hyde-Smith is still the favourite.

The issue of race has become a dominant issue in the campaign. On Monday the Mississippi Department of Public Safety said a number of nooses were placed around the state capitol building along with signs.

One of the signs read: "We need someone who respects lives of lynching victims." 

Republicans narrowly hung on to their majority in the Senate, the upper chamber of the Congress, after midterm elections this month, but they lost control of the House of Representatives.

The Mississippi seat will determine if the centre-right party has 52 or 53 seats in the 100-member chamber.

Trump has been dogged by his remarks about being a nationalist and accusations that this has emboldened white supremacists