Oz and Fetterman have a big debate Oct. 25. How many people will have voted by then?

As soon as Lt. Gov. John Fetterman accepted a debate invitation for Oct. 25, Mehmet Oz said their matchup would fall too late in the cycle — two weeks before Election Day — given people could start voting by mail as early as late September. He criticized Fetterman for declining earlier debates.

“John Fetterman has only agreed to one debate on October 25, which would take place after hundreds of thousands of absentee and mail-in voters begin casting their ballots,” Oz’s campaign said in a statement this week.

Fetterman’s campaign points to debates in years past that similarly fell about two weeks before the election, though in those contests there were two debates apiece in October.

The difference is many more people vote by mail now than in 2018, when Sen. Bob Casey and Lou Barletta held their first Senate debate Oct. 21.

And let’s also be honest about the politics: The candidate who is trailing almost always wants more and earlier debates. The one who is leading usually wants fewer. (As reporters, who want transparency that helps voters stay informed, we want to see candidates answering as many questions as possible.)

The televised debate in Harrisburg will be the one prime-time bout in a tight race for one of the most closely watched Senate contests in the country. It’s a former TV doctor against a stroke survivor with lingering speech challenges who has never been the most nimble debater. Typically, debates don’t move undecided voters too much, but individual moments can become flash points in the critical homestretch.

In more ways than one, it’s a made-for-TV matchup. So how many votes will be cast before it airs? We wanted to dig into it.

Most votes, even most mail ballots, will be cast after Oct. 25

Yes, the 2022 vote-by-mail numbers will look a lot different from the past, but we know from recent elections that most voters will still vote in person Nov. 8. Even most mail ballots tend to be cast in the final two weeks.

Look at 2020, the high-water mark for mail voting so far, thanks to both a presidential election and pandemic precautions: About two of five votes were cast by mail, and only about two of five of those votes were cast two weeks or more before Election Day.

In this year’s May primary, nearly 90% of mail ballots were recorded as received in the last two weeks. That means less than 5% of the vote was cast by the two-week mark.

Early voters tend to have already made up their minds

The kind of person who knows to request a mail ballot and sends it in early typically also is informed about the election and whom they support.

“There are obsessive people like me who return their ballot as soon as it arrives,” Democratic strategist J.J. Balaban said. “And people like my wife (an apolitical physician) who gets around to it 10-14 days before the election. The people like me who tend to vote immediately tend to be the least persuadable.”

The pool of persuadable potential Oz voters also is limited by the fact that Democrats remain much more likely to vote by mail than Republicans.

Debate might have higher stakes than most

With the lack of policy discussion we’ve seen in this race, this one-hour debate could be more influential than most.

It could be the first time Oz has to actually answer questions he’s consistently dodged, including:

• Will he support Sen. Lindsey Graham’s proposed 15-week abortion ban?

• Does he support a federal increase of the minimum wage, and by how much?

• What, if any, gun-control legislation does he support?

Oz has taken questions at campaign stops but has done very few one-on-one interviews outside of Fox News.

And for Fetterman, it will be one of the longest periods of direct questioning he’s faced. His campaign answers on his behalf when we ask policy questions, but at the debate, he will have to explain and defend those positions himself. How he communicates as he recovers from his stroke will be under scrutiny.

Late September polling indicated about 7% to 10% of voters were undecided in the Senate race.

They’re likely to make up their mind at the last minute, said Berwood Yost, pollster for Franklin & Marshall College.

“These are voters who don’t feel favorable toward either candidate but who think it’s important to vote,” Yost said. “So maybe some event turns the tide. It’s possible the debate may do that.”