Washington (AFP) - The final twist in the US midterm elections may emerge in Mississippi, where a Senate race that should have been easy winnings for Donald Trump's party is headed to a runoff, after multiple stumbles by the Republican incumbent.

The president is counting on the reliably Republican stronghold in the Deep South, where political battles are rarely on the national radar, to give him some breathing room with his majority in the 100-member Senate.

The runoff is looking increasingly important for Trump. On election night, he could have argued that his Republicans held their ground, as Democrats eked out a win in the House.

But as results were tallied in the weeks since, Democrats now appear to have won about 40 House seats, their strongest victory in decades, and managed to hold their own in the Senate despite an unfavorable election map.

Aware of the potential that the November 27 runoff goes badly for Republican Senator Cindy Hyde-Smith, Trump has scheduled election-eve rallies with her in two cities in Mississippi, a state Trump won by nearly 18 percentage points in 2016.

The tight Senate race in Florida flipping to the Republicans helped improve the narrative for Trump. If the party keeps Mississippi, Republicans will widen their narrow pre-election 51-49 majority to 53-47.

But out of nowhere, Mississippi may now be in play thanks to Hyde-Smith, who was appointed this year to fill a vacancy but has stumbled badly on the campaign trail in recent weeks.

In praising a supporter, Hyde-Smith said she would be "on the front row" if he invited her to attend a "public hanging."

Many bridled at the comment, all too aware that Mississippi lynched more African Americans than any other state.

Days later, she told a small crowd that laws that "make it just a little more difficult" for some liberal college students to vote are "a great idea."

Hyde-Smith, 59, said her lynching remark was an "exaggerated expression of regard," and that her voter suppression line was a joke.

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But her gaffes have clearly shaken up the campaign.

"She's trying really hard to turn this into a race," professor John Bruce, who chairs the political science department at the University of Mississippi, told AFP.

If Democratic challenger Mike Espy, a former congressman and the first African American to represent Mississippi in Washington in more than a century, manages to oust Hyde-Smith, "it will be an upset of the status quo the likes of which we haven't seen" in decades, Bruce added.

- Galvanizing Democrats -

The senator has made history herself as the first woman to represent Mississippi in Congress.

But her bungled comments are galvanizing Democrats. Mississippi is 37 percent black, the highest percentage of any US state, and her remarks were an affront to African Americans.

The state's voting patterns have largely been along party lines in recent decades, and Democratic candidates often find themselves hitting a ceiling of about 44 percent support, according to Bruce.

Trump's approval ratings are sagging, but he remains popular in Mississippi.

With high turnout among black voters the only likely path to victory for Espy, he has called in high-profile motivators.

US Senator Kamala Harris campaigned for Espy over the weekend, while Senator Cory Booker hit the trail with him Monday. Both are African American senators seen as potential Democratic presidential hopefuls in 2020.

Mississippi's Senate race is headed to a runoff because none of the four candidates received more than 50 percent of the vote on Election Day.

Hyde-Smith split the Republican vote with a far-right conservative whose supporters are likely to consolidate behind Hyde-Smith.

Espy, 64, has insisted he will unify voters. He has pledged to work to keep the US health law that protects coverage for people with pre-existing conditions.

In eyeing an upset, Espy and supporters can turn to neighboring Alabama, where Democrat Doug Jones stunned the political world last December by defeating a conservative Republican.

They can also seek inspiration from California's Orange County.

The area outside Los Angeles was a conservative bulwark for decades. But thanks in part to shifting demographics, Democrats swept all seven congressional races in the county this month, leaving it without a Republican representative in Congress.