UPDATED below with comments from Sestak.

WASHINGTON – A nurse's union endorsed Democrat Katie McGinty in the Pennsylvania U.S. Senate race Wednesday – two years after first backing one of her party rivals, Joe Sestak.

The executive director of the union, the Pennsylvania Association of Staff Nurses and Allied Professionals, said Sestak had failed to support with the nurses, prompting them to pull their endorsement.

"We were just very disappointed in Joe Sestak in the fact that he steadfastly refused to stand publicly with nurses who were in a difficult situation," said Bill Cruice, executive director of a union that represents around 5,500 nurses and hospital staff across the state, including about 3,500 in the Philadelphia area.

Cruice pointed to a Sept. 2014 rally at Crozer-Chester Medical Center when nurses there were on strike. Sestak, he said, declined to attend, though other public officials from both parties did so, he said. The union warned Sestak's camp then that it could cost him their support, he said.

"We supported Joe Sestak when he was in Congress," Cruice said in an interview. "For our members and for the nurses, there's just a real sharp contrast in willingness to stand publicly with nurses and their issues and what they're most concerned with."

UPDATE: Sestak said Cruice "is just stating the truth" and he had a committment outside of Philadelphia that day.

"I couldn't be there the day in 2014 that Bill had asked (as I was committed out of Philadelphia) and he said if I didn't show up he'd pull the endorsement -- and I understood that," Sestak said in a statement e-mailed by an aide. "I continue to believe that nurses are the chief petty officers of our nation's medical field, the ones that truly keep it all running. It's why each year, at the fundraiser I host for the Brain Tumor Institute, I invite two nurses who helped save my daughter Alex's life."

McGinty, Cruice said, recently attended a press conference for nurses attempting to form a union at Eagleville Hospital and met with the union's executive board.

"We understand in talking to her and watching her in action she will be a much more dynamic and from our perspective effective candidate for the Democratic Party," Cruice said, adding that the union's main goal is unseating the incumbent, Republican Sen. Pat Toomey.

To that end, the organization endorsed Sestak in Sept. 2013, saying then that he would be "a Senator with the courage to work on behalf of the people he represents."

McGinty, Sestak and Braddock Mayor John Fetterman are competing in the Democratic primary to challenge Toomey next year.

McGinty, the former chief-of-staff for Gov. Wolf, has won the most support from the Democratic establishment, leading Sestak to derisively brand her as the insider's pick.

McGinty, in a news release, pledged to "work to ensure that every American has access to affordable health care" by supporting President Obama's Affordable Care Act and working with "the health care community."

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