BOSTON — Critics who say unions block education reforms and make it virtually impossible to fire bad teachers will offer 10 instructors it deems the nation's worst $10,000 to quit their careers.

The Center for Union Facts, a Washington-based nonprofit, launched a campaign Tuesday spending $1 million on ads and a billboard in New York's Times Square. It also says it's starting a Web site with data documenting how far unions go to protect bad teachers.

It's also inviting nominations for a contest to determine the nation's worst unionized teachers. The "winners" will be offered $10,000 each if they permanently resign or retire from any career in education — if they sign a release agreeing to have their name and the reasons for their selection published by the group.

Rick Berman, the center's executive director, declined to name any of his group's supporters. He said the organization has spent about $6 million since its founding in February 2006 and that it opposes union corruption and intimidation, not unions in general.

The head of the nation's second-largest teachers union, the American Federation of Teachers, called Berman an "ethically challenged attack dog."

"Berman has a record of using hidden funders to attack groups that contribute a great deal to society," said Edward McElroy, president of the federation. "Now he is coming after teachers at a time when most Americans support education and want to make improving education a top national priority."

Reg Weaver, a spokesman for the largest teachers union, the National Education Association, said school districts' evaluation policies include strict criteria to ensure teachers face consequences for poor performance.

"This union does not support a person's incompetence. This union supports a person's right to due process," Weaver said.

The campaign targets about two dozen districts from Boston to Anchorage, Alaska. Hawai'i is not among the districts listed.