Mayor Edward I. Koch, his popularity at a new low, would lose at every level of a mayoral election, and former U.S. Attorney Rudolph Giuliani would win, according to a poll released Sunday.

Among Democrats, Manhattan Borough President David Dinkins would beat Koch in a primary if it were being held now, according to the poll. The primary is set for September and the general election for November.The telephone poll, conducted March 3-7 by the Gallup Organization for New York Newsday, surveyed 1,012 New Yorkers.

Six months before the primary, 50 percent of those polled disapproved of how the three-term mayor does his job, up five points in three months. Thirty-one percent approved of the mayor's performance, down from 37 percent in December.