Call it an un-endorsement by the Philadelphia Federation of Teachers.

The union on April 15 endorsed Montgomery County Commissioners Chairman Josh Shapiro in Tuesday's Democratic primary election for attorney general.

On Monday, PFT spokesman George Jackson said the union is not endorsing any candidate in that election and declined to comment on or even acknowledge the endorsement of Shapiro 10 days ago.

What happened?

While the PFT won't say, Shapiro's foes cite a campaign donation as the source of the controversy.

Shapiro received a $100,000 donation from Students First PA, a political action committee that advocates for school vouchers and charter schools, efforts adamantly opposed by the union representing public school teachers in the city.

The timing of how that donation was made and reported has raised questions about potential political gamesmanship by Shapiro's campaign.

Students First PA reported to the Pennsylvania Department of State on March 15 that it had made the donation to Shapiro on Feb. 25.

Shapiro's campaign waited two weeks to record the donation, on March 15. As a result, it did not have to be reported to the state until April 15, the day the PFT endorsed him.

Shapiro spokesman Joe Radosevich said the campaign had not heard from the PFT that it had revoked the endorsement.

And he said, "Our opponents are concocting conspiracies" about Shapiro's delay in reporting the donation, which he called common in campaigns.

Allegheny County District Attorney Stephen Zappala Jr., another Democrat in the primary, on Sunday stoked the union unrest, noting that Shapiro has taken Students First PA donations before.

According to Pennsylvania Department of State records, Shapiro took $25,000 from Students First PA on March 6, 2015, and $25,000 on Dec. 5, 2012.

Zappala said the PFT had "rescinded" its endorsement due to the most recent donation.

"We are not surprised," Zappala campaign manager Marty Marks said in an email. "Josh Shapiro has never met a political campaign contribution he didn't like."

John "Johnny Doc" Dougherty, leader of Local 98 of the International Brotherhood of Electrical Workers, in a letter to other unions Sunday, said Shapiro's "most recent moves and decisions have me scratching my head." Local 98 has endorsed Zappala.

PFT president Jerry Jordan did not respond to requests for comment. In the April 15 endorsement, which Shapiro's campaign said was circulated with the PFT's approval, Jordan said, "Josh understands that public education is the cornerstone of our society, and he's committed to using all of the powers of the Attorney General's Office to protect the constitutional rights of Pennsylvania's students."

Zappala's campaign on Monday said the race with Shapiro is "a statistical dead heat."

Shapiro on Monday touted a Harper Polling survey that showed him with an 18-point lead among likely Democratic voters. The poll showed Shapiro at 41 percent, Zappala with 23 percent, and Northampton County District Attorney John Morganelli, the third Democrat in the race, with 16 percent.