The 2016 election will finally end on Saturday in Louisiana.

More than three months after the first absentee ballots were sent out in North Carolina, voters will head to the polls in the Pelican state’s runoff for US Senate.

Democratic underdog Foster Campbell will face off against Republican John Kennedy in a race that will determine just how much breathing room President-elect Donald Trump will have in the Senate. Currently, Republicans are due to hold 51 seats in January; Kennedy’s election could give them 52, thus giving the GOP a cushion if they face any defections on the raft of legislation and nominations to come in 2017.

By contrast, an upset victory by Campbell would mean Democrats would only need one Republican defector to force a tie vote which Vice-President Mike Pence would need to be present to break, and only two defectors to defeat any GOP legislative initiatives.

The stakes were high enough for Trump to hold a rally with Kennedy in Baton Rouge on Friday, just a day before the runoff. The president-elect’s trip followed a campaign rally Pence held with Kennedy last week. In a state that Trump won overwhelmingly, he urged the crowd to vote for Kennedy. “If he doesn’t win, I’ve got myself a problem in Washington,” he said, adding: “It’s pretty close,” in reference to the slim Republican majority in the Senate. “We need John in Washington, not only for the vote, but for leadership and everything,” Trump said.

The president-elect took the stage after Kennedy railed against Campbell’s ties to Hillary Clinton in front of a cheering crowd in the conservative southern state. “He supported Secretary Clinton. In fact, I don’t mean to be rude, but if you took Secretary Clinton upside down and shook her, Foster Campbell would fall out of her pocket.”

Mike Pence stumps for John Kennedy in New Orleans last week. Photograph: Gerald Herbert/AP

Campbell has slid under the national radar and has tried to minimize his conflicts with the president-elect. In a closing television ad in which the state’s newly elected Democratic governor, John Bel Edwards, spoke directly to the camera on behalf of Campbell, he said the Democratic candidate “will stand with the new president when he’s right for Louisiana but has the courage to say no when he’s wrong”.

However, the race has captured the attention of a number of Democrats dejected by Trump’s surprise win in November as the last-ditch chance for progressives to notch an electoral victory before the president-elect takes office. The result was that Campbell outraised Kennedy by more than $1m in recent weeks with the support of a number of celebrities.

But Campbell still faces long odds. John Brabender, a veteran Republican political consultant who has worked on a number of races in Louisiana, told the Guardian: “I just don’t think it’s his race.” He noted that “early voting in the state was anemic” and pointed to a highly competitive runoff between two Republicans also taking place on Saturday in a deep red congressional district that is likely to drive up GOP turnout.

He also noted that Kennedy, a former Democrat, was positioned to do better among African American voters than many other Republicans, a key advantage in a state where a third of the population is black.

The runoff is happening in December because of Louisiana’s unique “jungle primary” system. The state does not have primary elections for any office except the presidency. Instead, all candidates for office run on the same ballot in November. If one receives a majority of the vote, that person is declared the winner. If not, the top two candidates advance to a runoff. In November, 24 candidates ran for Senate, including two sitting Republican congressmen. Then Kennedy, the state treasurer, finished on top with more than 25% of the vote, with Campbell, an elected member of the state’s public utility regulator, finishing second at 17.5%.

Polls in Louisiana will be open from 7am to 8pm on Saturday, when the final votes of the 2016 election will be cast.