Several thousand janitors, doormen and other employees who help clean and operate New York City’s office buildings voted overwhelmingly on Wednesday to authorize a strike on Jan. 1 if the city’s real estate industry does not grant them the substantial raise their union is seeking. Mike Fishman, the union’s president, said that a large raise was warranted because office rents in Manhattan have jumped by more than 25 percent in the past year. Negotiators for the building owners say that the workers are already the nation’s highest paid and that the industry could be hurt by the shaky economy.