Although Fetterman, Pennsylvania’s current lieutenant governor, enjoyed a double-digit lead in some polls taken over the summer, Oz has rapidly retaken ground while many midterm elections across the country have swung toward Republicans.

The candidates are vying for the seat of retiring Republican Senator Pat Toomey. The contest is one of the closest midterm elections in the country and could be critical in the battle to control a Senate that is currently split evenly between parties.

Two polls released on Monday suggested that Fetterman is maintaining a small advantage over the celebrity doctor. A survey from Rasmussen Reports found that 45 percent of respondents favored Fetterman, while 43 percent preferred Oz. A CNN/SSRS poll showed Fetterman with a 51 percent to 45 percent lead over the Republican.

The Democrat’s advantage in the Rasmussen poll falls within a 3 percent margin of error. Fetterman’s lead in the CNN/SSRS poll is just outside of a 4.6 percent margin of error.

The Rasmussen poll was conducted among likely voters over the phone on October 19 and October 20, while the CNN/SSRS survey was conducted among likely voters by phone and online between October 13 and October 17.

Another recent poll of likely voters, taken by Echelon Insights between October 18 and October 20, found that Fetterman had a 3 point lead over Oz—well within a 4.8 percent margin of error. The candidates were tied in an Insider Advantage poll conducted on October 19.

Only one recent poll has shown Oz with an advantage over Fetterman. A poll conducted by Wick between October 8 through October 13 shows the Republican with a 49 percent to 45 percent lead over the Democrat.

A RealClearPolitics average of polls shows Fetterman with a 2.2 percent lead over Oz as of Monday evening. However, the site is also predicting that Oz will win the race, based on some previous polling underestimating support for Republicans.

Polling analysis site FiveThirtyEight projects that Fetterman is “slightly favored” to win the election, with likelihood of 60 percent to Oz’s 40 percent.

Dan Mallinson, an assistant professor of public policy and administration at Penn State University-Harrisburg, told Newsweek that he was “not terribly surprised that the polls have been tightening,” adding that the race would “likely be a close one.”

“Oz has been one of the weaker of the Republican candidates, but partisanship is a strong factor in voting and Pennsylvania is a pretty evenly divided state,” Mallinson said. “Republicans may not have liked Oz that much, but I’m guessing they will vote for him… It’s going to be harder for Democrats to convince folks who think things are going the wrong way to vote for them.”

The candidates are scheduled to take part in a debate in Harrisburg on Tuesday night, presenting an opportunity to potentially sway the small number of Pennsylvania voters who remain undecided.

Fetterman, who initially declined to debate Oz, agreed to take part last month following extended negotiations and the Oz campaign’s repeated questioning of Fetterman’s ability to serve after a stroke that he suffered in May.

Mallinson said that both candidates will need to perform well in the debate to have any hope of swaying voters, albeit “in different ways.”

“Oz needs a good debate on the issues, otherwise keeps looking like a rich man who wants to be Senator, but doesn’t stand for much,” he said. “Fetterman does need a reasonably good debate as it pertains to his health… I think for him he needs to not get flustered or frustrated by his ongoing auditory processing issues. He needs to own it.”

The debate has the potential to be a tense affair. In addition to disputes over stark policy disagreements, the candidates have also launched personal attacks on each other that experts recently described to Newsweek as “juvenile.”

While the election will be held exactly two weeks after the debate, early voting in Pennsylvania began on September 19.

Brittany Yanick, communications director for the Oz campaign, told Newsweek in a statement that “Dr. Oz is criss-crossing the commonwealth and meeting with Republicans, Democrats, and Independents who want to see a change from the failed policies of the past.

“We are going to win in November because Pennsylvanians can’t afford a Bernie Sanders socialist like John Fetterman who wants to release 1/3 of PA inmates, decriminalize all drugs, and eliminate life sentences for murderers,” Yanick added. “John Fetterman is the most pro-murderer candidate in the nation - the choice is clear.”

Newsweek reached out to the Fetterman campaign for comment.

Update 10/24/22, 11:13 p.m. ET: This article was updated with additional information and background.