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.