NEW ORLEANS  For two years as a presidential candidate, Barack Obama addressed educators gathered for the summer conventions of the two national teachers’ unions, and last year both groups rolled out the welcome mat for Education Secretary Arne Duncan.

But in a sign of the Obama administration’s strained relations with two of its most powerful political allies, no federal official was scheduled to speak at either convention this month, partly because union officials feared that administration speakers would face heckling.

The largest union’s meeting opened here on Saturday to a drumbeat of heated rhetoric, with several speakers calling for Mr. Duncan’s resignation, hooting delegates voting for a resolution criticizing federal programs for “undermining public education,” and the union’s president summing up 18 months of Obama education policies by saying, “This is not the change I hoped for.”

“Today our members face the most anti-educator, anti-union, anti-student environment I have ever experienced,” Dennis Van Roekel, president of the union, the National Education Association, told thousands of members gathered at the convention center here.