It could be one for the ages.

The turnout for the Big Apple mayoral race is hitting historic highs, with nearly 2 million voters expected to cast their ballots by the end of Election Day Tuesday, leading pollsters tell The Post.

The dizzying numbers, already the highest in the five boroughs in decades, could see more moderates heading to the polls — which could spell trouble for lefty Democrat Zohran Mamdani, the frontrunner, as he tries to fend off a recent surge by former Gov. Andrew Cuomo, the experts said.

Guardian Angels founder Curtis Sliwa is running on the Republican line.

“What’s driving it is we have the most interesting, hotly contested general election for mayor since 2001,” said Evan Roth Smith, a pollster with Slingshot Strategies. “And New Yorkers turn out when there’s actually something to vote about.”

At least 1.9 million voters are expected to vote in the race, topping the 1.5 million who voted when Republican Michael Bloomberg defeated Mark Green in 2001, and believed to be the highest Big Apple turnout since nearly 2.5 million headed to the polls in 1969 to elect liberal John Lindsay.

This year could come close.

“You are really going to end with another 100,000 today,” Stephen Graves of Gotham Polling said Sunday. “You have to figure we are going to have at least 700,000 votes, and that would push the turnout to 1.8 or 1.9 million, 1.94 million. That is going to be over the 2022 vote in the congressional election.

Graves’ early voting prediction proved spot-on, according to an unofficial tally released by the city Board of Elections.

By Sunday night – when early voting ended – 735,317 New Yorkers had flocked to the polls.

“By the numbers, it’s surprising,” Graves said. “It’s going to be between a congressional election and a presidential election. This one, being near 2 million, is huge.”

He said while a boost in younger voters is expected to help Mamdani, a socialist Muslim, the higher turnout could help Cuomo, a traditional Democrat running on an independent line.

“As the turnout gets larger, it leans more moderate and brings in the independents,” Graves said. “That benefits Cuomo because he was getting more independent while the vast majority of Mamdani’s voters were Democrats.

“But Mamdani can absolutely win with just Democrats,” he added.

He said it’s unlikely that Mamdani will get a majority of all the votes in the three-way race, “so it’s about how that gets split up between Cuomo and Sliwa.”

Mamdani’s surprise primary win over Cuomo and Mayor Eric Adams rattled political circles, with Republicans and traditional Democrats worried about the Queens state assemblyman’s socialist views.