The Citizens Union has backed away from a push to give fixed terms to members of the citywide school board, following lobbying from Mayor Bloomberg and President Obama’s secretary of education, Arne Duncan, according to sources familiar with the watchdog group’s stance.

Bloomberg has vigorously opposed fixed terms. He says he needs to be able to dismiss school board members at his pleasure in order to have real control over the public schools.

Members of the Citizens Union had previously voted to endorse fixed terms. But the position the Citizens Union, a nonprofit good-government group, will recommend tomorrow backs away from the fixed-terms power check. As a compromise, it would force the mayor to give 90 days’ notice before dismissing a board member, sources said.

Bloomberg reached out to the group after it briefed City Hall on the first proposal last week, urging board members to reconsider their stance. The group subsequently re-started its process of debating and voting on a position, sources said.

Duncan also weighed in during that period, writing a personal letter urging the group to preserve the mayor’s power over the schools, sources said. Duncan has previously said he supports mayoral control as a way to improve urban schools.

Teachers union president Randi Weingarten and the education historian Diane Ravitch, who has pushed for checking the mayor’s power with an independent school board, also made pitches to Citizens Union members, sources said. A member of the group who is a stronger critic of mayoral control recruited Ravitch to speak at a meeting yesterday in a last-minute pitch to counter the push by Bloomberg and Duncan.

The Citizens Union will announce the proposal at a press conference on the steps of Tweed Courthouse tomorrow at noon. The executive director of the Citizens Union, Dick Dadey, declined to comment in a telephone interview. “We’ll make that information available tomorrow at our press conference,” he said.

A spokesman for Bloomberg, Jason Post, also declined to comment. A spokeswoman for Duncan did not immediately have a response.