Chanting “No ultimatums!” New York University graduate student teaching assistants and supporters rallied in front of Bobst Library on Washingon Square South on Friday. Top, John Sweeney, A.F.L.-C.I.O. president, encouraged the strikers, while, below, an inflatable giant rat, the symbol of antiunion practices, bared its fangs nearby. Five days earlier, university President John Sexton, in a 1,400-word e-mail statement, announced that teaching assistants who struck past Dec. 5 would not be allowed to receive stipends or teach classes next semester. “However strongly felt a graduate assistant’s act of conscience may be, it should not be pursued any longer at the expense of undergraduates,” Sexton said. Sexton has since extended the deadline to Dec. 7. But on Dec. 6 union spokesperson Susan Valentine said it’s an arbitrary extension. “There was no offer for negotiations,” Valentine said. “We’ll still be on strike tomorrow.” In 2000, the National Labor Relations Board for the first time recognized graduate students as workers, and N.Y.U.’s G.A. union became the first in the country at a private university. But the N.L.R.B. reversed its position on graduate students in 2004, following which N.Y.U. refused to recognize the G.A. union’s contract after it expired in August of this year. The almost 1,000 G.A.’s have been on strike since Nov. 9. N.Y.U. offered them a nonunion contract with full medical benefits, free tuition and a $1,000 annual stipend increase until 2008. The Graduate Student Organizing Committee has vowed to continue the walkout until N.Y.U. bargains collectively and negotiates a contract with the union.