Zohran Mamdani walked across the Brooklyn Bridge to New York’s City Hall early Monday morning holding a banner that seemed to answer the president who has opposed him and the portions of the Democratic Party that have been slow to embrace him. “Our time is now,” it said. Mamdani faces former Gov. Andrew Cuomo, who is attempting a political comeback as an independent after losing the Democratic primary, and Republican Curtis Sliwa in the mayoral race — one of a handful of contests around the country Tuesday that will draw copious attention.