After $18 million in campaign spending and a five-week battle for the heart and soul of Chicago, it’s finally up to voters to decide whether to make Paul Vallas or Brandon Johnson the city’s 57th mayor.

Try not to hold your breath for the final outcome. The winner may not be known for days.

“I highly doubt either camp will concede on election night because up to 100,000 votes may not be counted when we go to bed on election night,” said pollster Matt Podgorski of M3 Strategies, whose polling correctly placed the top four finishers in Round One of the mayoral sweepstakes within roughly half a percentage point.

“Folks who have a pretty good idea of which way the late ballots break will be able to have a pretty good idea of who’s gonna win on election night,” Podgorski added. “But no one’s going to concede if there’s enough outstanding ballots for it to make a difference.”

Election Board spokesperson Max Bever agreed even a four-percentage-point gap between Vallas and Johnson on Tuesday night might not be safe.

“We’ll probably have to see how those vote-by-mail ballots come back” before either camp concedes, Bever said Monday.

Every ballot returned by 7 p.m. Monday will be counted on election night. The rest will have to wait.

Technically, the board has until April 18 to count all vote-by-mail ballots postmarked by 7 p.m. Tuesday. By the close of business Monday, Bever expected to have as many as 110,000 vote by mail ballots counted. With 200,000 applications, that still leaves up to 90,000 outstanding votes — but probably less than that, based on the first round of voting, Bever said.

“Not all of those might be coming back. Of the 100,000 outstanding that we had as of election day for Feb. 28, we only got 52,000 of those back over the next two weeks. So, it’ll probably be a similar return rate,” Bever said.

“It’s safest to say it’s that first week that we get the majority of vote by mail ballots back. I’d say if it’s close on election night, we’ll know or have the majority of vote by mail ballots back by April 11.”

Podgorski said there’s no question late-arriving mail-in ballots will break heavily for Johnson.

“His base is younger, more progressive people and those people tend to turn their ballots in late, historically. They’re just more susceptible to GOTV [get out the vote] efforts. They’re the ones who are less likely to vote without a strong push,” Podgorski said.

Anthony Vazquez/Sun-Times

“Campaigns do their math. They say, ‘Who hasn’t turned in their ballots of those who requested them?’ And then, they push, push, push to get ’em in. Both camps do that. Overall, Brandon has a slight edge on mail. But late mail, it could break as much as 2 to 1.”

Podgorski speaks from experience on the impact of late-arriving mail-in ballots. He ran for the Cook County Board as a Republican on Nov. 8 and lost that election to Maggie Trevor because of votes by mail counted after election day.

“When I went to bed on election night, I was winning by 1,200 votes. I lost seven days later. Not just vote-by-mail. Late vote-by-mail,” he said.

Last week, Vallas’ campaign manager and chief strategist told the Sun-Times they expected the winner not to be known until days after the election because the number of mail-in ballots was almost certain to be bigger than the margin separating Vallas and Johnson.

Johnson’s camp was still holding out hope for a definitive winner on election night. They noted similar fears in Round One never materialized.

Vallas spent $20 million on the race for mayor (his second), $13 million of it for the runoff alone, thanks to an avalanche of campaign contributions from business leaders.

Anthony Vazquez/Sun-Times

Johnson raised $9 million overall, $5.1 million of it in the runoff. Most came from the Chicago Teachers Union and its affiliates as well as SEIU Locals 1 and 73, SEIU Healthcare and ASCME Council 31.

His trump card could be 2,000 get-out-the-vote foot soldiers from those unions. Vallas will have about half that from the 26 unions endorsing him.

With severe weather possible, the election board is urging Chicagoans who haven’t voted already to cast their ballots for mayor and in 14 aldermanic runoffs as early as possible Tuesday.

With all of the talk about the racial divide between the candidates and the gender gap between progressives and conservatives, Podgorski talked more about the generation gap between Vallas and Johnson voters.

He expects the mayor’s race to be decided by the “turnout ratio of younger vs. old.”

“Do we get the normal age distribution of turnout? Or is the turnout gonna be a little bit older or a little bit younger than usual? ... Brandon’s voters are overwhelmingly young,” Podgorski said.

“Typically, 32% of the electorate is 45 and under. Brandon needs to drive that number up to 35 or 36%. Paul wants to do the opposite. He wants the older vote to be more than 68%.”

Vallas started his final day greeting CTA commuters at the 95th Street station in the morning. He was joined at a Roseland donut shop by retired U.S. Rep. Bobby Rush and former mayoral challengers Sophia King and Ja’Mal Green and by Ald. Anthony Beale (9th) and Ald. Derrick Curtis (18th).

“I’m not paying attention to any of the noise going on out there. I’m not paying attention to the polls. I’m just driving on. I see that finish line,” said Vallas, 69.

Johnson, 47, jammed 10 stops into his frenetic final day schedule. It included a prayer service at Rainbow/PUSH, a visit to Olive-Harvey College, a late canvass lunch on the West Side and visits to early voting sites in three predominantly Hispanic wards.

“I was polling at 2.3%. Why worry now?” Johnson said.

“I have confidence that the people of Chicago want a better, stronger, safer city. And I’m taking that confidence into tomorrow. And we’re gonna take that confidence all the way to the 5th floor” of City Hall.