Marshalling harsher words than she has used in months, the president of the teachers union, Randi Weingarten, is lashing out against the city, decrying a project to crack down on bad teachers.

The Bloomberg administration’s $1 million initiative aims to help principals either improve or fire unsatisfactory teachers. Principals judged nearly 1,000 tenured teachers unsatisfactory last year, but only 10 were removed from the system. During contract negotiations last year, Ms. Weingarten signed onto part of the plan, a program for teachers to help other teachers improve.

Ms. Weingarten said another part of the plan — for a new team of attorneys the city is billing the Teacher Performance Unit, which will help principals build cases against tenured teachers they believe are incompetent — shocked her when she read its details Wednesday night. “What they said to me about what this unit was and what was in this memo was the difference between night and day,” she said.

Ms. Weingarten called the team a “teacher gotcha unit.”

The program was announced in a memo to principals the night before the results of a national test showed city students making few significant gains in reading and math.

Ms. Weingarten said the timing appeared to have been deliberate. “The moment they have some bad news, they immediately resort to blaming the teachers,” she said. “I’m very disappointed that they don’t take any responsibility for educating kids.”

The schools chancellor, Joel Klein, said he thought the test scores, on the National Assessment of Educational Progress, were good, and he defended his move to boost principals’ capacity to fire teachers. “Any responsible person in my position would take these measures,” he said.

He added, “As a unit our teachers are doing a phenomenal job, but that doesn’t mean everyone is doing a phenomenal job.”

The Teacher Performance Unit was first reported in the New York Times yesterday.