For New York University’s self-assured president, it’s a moment of uncertainty.

Starting today, John Sexton will have to manage without a sizable portion of his teaching force. Hundreds of graduate students – who teach, head discussion groups, grade term papers, mark exams, and hold office hours – are going on strike, refusing to perform any of those duties until the university recognizes their union.

Both supporters and critics of the strike agree that Mr. Sexton’s options are limited and that his ability to minimize academic disruptions are somewhat out of his control. To weather this storm, Mr. Sexton is counting on support from faculty members – who are sharply divided over whether graduate students should be allowed to form a union – and a low level of student participation in the strike. About 1,000 graduate students serve as teaching assistants every semester.

While the university has threatened to punish graduate students who abandon their teaching duties, NYU officials are aware that any retaliation may backfire and win the strikers more sympathy. More than a dozen departments have passed resolutions opposing any penalties against students.

While some departments, particularly sciences, math, economics, and philosophy, have plans to fill in for their teaching assistants, others may not be as cooperative. NYU professors sympathetic to the strikers have said they do not intend to do any ex tra grading or teaching and have received no instructions from department chairmen to cover for their teaching assistants.

“Everyone is basically playing a wait-and-see game,” said Anthony Movshon, a professor at NYU’s Center for Neural Science who is a strong opponent of graduate student unionizing. For NYU officials, “There aren’t a lot of things they could do to make things better,” he said.

With both sides battening down, some are predicting a strike that will be measured in months. The graduate stu dents have said they won’t call off their strike unless the university returns to collective bargaining.

Mr. Sexton, who cut off talks with the union after gaining permission from the National Labor Relations Board, has given no indication that he plans to back down. Yesterday morning, the president met with professors and reiterated his opposition to bargaining with the United Auto Workers, which represented the union until the contract expired this summer.

“I don’t know how you get out of this one,” the director of the National Center for the Study of Collective Bargaining in Higher Education and the Professions at Hunter College, Richard Boris, said. “It’s a real hard one.”