UFT boss Michael Mulgrew escalated the war over teacher evaluations Thursday, threatening that Gov. Cuomo will “rue the day” he proposed using student test scores to account for 50 percent of the grades.

“If you want to keep emphasizing test scores, we’re not going to sit quietly,” Mulgrew said at a union-sponsored rally.

He said schools will suffer, fighting will increase and “we’re going to make sure the governor rues the day that he did it.”

Mulgrew, who was speaking on the steps of Brooklyn’s PS 10 at one of dozens of “Protect Our School” rallies across the city, railed against Cuomo and Albany lawmakers for playing “political games” at the expense of students.

Cuomo’s supporters charged the union with confusing matters to “protect its own interests.”

Meanwhile, at City Hall, charter-school supporters ripped Mayor de Blasio for delays in upgrading failing public schools.

His “renewal schools program is seven months in and he has not acted on any of the components,” said Joe Herrera, of Families for Excellent Schools.