MINNEAPOLIS — For years, the unionized teaching profession opposed few ideas more vehemently than merit pay, but those objections appear to be eroding as school districts in dozens of states experiment with plans that compensate teachers partly based on classroom performance.

Here in Minneapolis, for instance, the teachers’ union is cooperating with Minnesota’s Republican governor on a plan in which teachers in some schools work with mentors to improve their instruction and get bonuses for raising student achievement. John Roper-Batker, a science teacher here, said his first reaction was dismay when he heard his school was considering participating in the plan in 2004.

“I wanted to get involved just to make sure it wouldn’t happen,” he said.

But after learning more, Mr. Roper-Batker said, “I became a salesman for it.” He and his colleagues have voted in favor of the plan twice by large margins.

Minnesota’s $86 million teacher professionalization and merit pay initiative has spread to dozens of the state’s school districts, and it got a lift this month when teachers voted overwhelmingly to expand it in Minneapolis. A major reason it is prospering, Gov. Tim Pawlenty said in an interview, is that union leaders helped develop and sell it to teachers.