Mississippi will vote in the final midterm of the 2018 U.S. election cycle on Tuesday, a Senate runoff poll marred by a racial controversy that the Republicans are desperate to win.

Senator Cindy Hyde-Smith faces off against Democrat Mike Espy, who is bidding to be the first African American to represent the state in the Senate since the reconstruction era.

The state is traditionally a Republican stronghold, but Hyde-Smith has made a series of blunders on the campaign trail to give Espy an outside chance of causing an upset.

A Democratic win would peg the Republican majority in the Senate at 52-48, just one up on the present session. The Democrats flipped the House of Representatives and seven governor's mansions in the November 6 elections.

Weekend polling put the Republican 10 points ahead, but the party is nevertheless nervous – it has poured money into the state and U.S. President Donald Trump held two rallies there on Monday alone.

The concerns are a consequence of controversial comments from Hyde-Smith. She was recorded saying she would be "on the front row" if one of her supporters "invited me to a public hanging," widely seen as alluding to Mississippi's history of lynchings. The state has a higher proportion of African Americans than any other in the U.S.

Espy, a former congressman who served as agriculture secretary in the Clinton administration, accused his opponent of having "rejuvenated old stereotypes." The Democrat is no stranger to controversy himself, having resigned from his role in Bill Clinton's cabinet after being indicted for illegally accepting gifts. He was later acquitted on all charges, but the issue has been prominent in negative advertising. 

Trump said Hyde-Smith "misspoke" and claimed she had apologized, although she has only said sorry to "anyone offended" by her comment. Meanwhile, police launched an investigation on Monday after two nooses were found hanging from trees at the state capitol in Jackson.

Hyde-Smith has also been recorded saying it would be "a great thing" to suppress votes of liberal students, and a number of revelations about her past – including attempts to name a highway after Jefferson Davis, president of the slavery-backing confederacy – have led to major businesses including AT&T and Walmart to ask for their donations to be returned.

Trump went all in on the Republican candidate on Monday, however, praising her as "a very, very special woman who's going to do a special job" opposing "radical socialism and open borders."

A win in Mississippi would only extend the Republican Senate advantage by one, but putting the margin at 53-47 could be decisive as Trump looks to rejig his top team.

The president has already jettisoned attorney-general Jeff Sessions and is yet to name a permanent replacement – that would require Senate confirmation. 

He is also widely expected to make further changes to his Cabinet in the coming weeks and is still to name a new U.S. ambassador to the UN following Nikki Haley's resignation. 

The extra Republican vote in the Senate could make all the difference, with Susan Collins and Lisa Murkowski among the party's senators who have wavered over Trump picks over the past two years.  

The runoff is taking place because no candidate gained a majority in the November 6 election, in which far-right Republican Chris McDaniel gained 16.4 percent of the vote. Hyde-Smith finished first with 41.3 percent, followed by Espy on 40.9 percent.