Breaking with AFL-CIO, Patrick backs plan to curb teachers’ seniority.

Dismissing the AFL-CIO’s charge that the proposal is an attack on collective bargaining rights, Governor Deval Patrick on Monday expressed support for a deal backed by the state’s largest teachers union that would curb teachers’ seniority protections.

The agreement, brokered between the Massachusetts Teachers Association and Stand for Children, an advocacy group, would prioritize teacher evaluations over seniority rights in school staffing decisions.

Backers say that will make it easier to dismiss ineffective teachers and reward those who perform well. Opponents say it will erode hard-won protections for veteran teachers.

If the pact does not pass the Legislature by July 3, Stand for Children has said it will place a more sweeping question on the November ballot, an initiative that polls indicate is likely to pass.

The Teachers Association has said it would rather give up some of its members’ seniority rights than fight a bitter and costly ballot war this fall.

But the AFL-CIO of Massachusetts and the American Federation of Teachers Massachusetts have blasted the agreement, calling it an assault on workers’ rights.

Labor’s split on the issue has put Patrick in a difficult position. A Democrat, he considers himself a friend of unions. But he has also sought to make educational improvement a priority.

On Monday, Patrick made clear he sides with the Teachers Association, not the AFL-CIO.

He said his administration had already been working on a plan similar to the agreement backed by the association and Stand for Children.

“It’s not taking away anything,’’ Patrick said, dismissing the argument that the deal would cut collective bargaining rights. “It’s about how we make sure we get the very best and highest-performing teachers in front of the kids who need them.’’

Patrick said his only worry is how to pay for the agreement. The deal calls for the state to funnel an additional $13 million to school districts to help them use a new teacher evaluation system that will guide staffing decisions.

“I celebrate and congratulate MTA and Stand for Children, but they have cut a deal that requires somebody else to come up with the money, and we’re trying to figure out how to do that right now,’’ he said.

Patrick did criticize Stand for Children for pushing the ballot initiative, even though the tactic compelled the teachers’ association to enter into negotiations with the organization.

“The ballot initiative was ill-timed and unnecessary,’’ he said. “And I’ve said that to Stand for Children.’’