Americans for Prosperity Foundation Chairman David Koch speaks in Orlando, Fla., in 2013. (Phelan M. Ebenhack/AP)

A major labor union is cutting ties with the United Negro College Fund because of the group's connections to the billionaire industrialist Koch Brothers, who have put their wealth behind supporting conservative causes and candidates for office.

Lee Saunders, president of the American Federation of State, County and Municipal Employees, penned a letter this week to UNCF President and CEO Michael Lomax informing him that he is ending the collaboration between the two groups on a joint scholarship program.

"Like many supporters of the UNCF, I was deeply troubled by your decision to accept $25 million from David and Charles Koch. But I assumed that in accepting those funds you were in no way supporting or lending the name of the UNCF to the political or social causes or substantive views of the Koch brothers," writes Saunders. "So I was truly stunned to learn that less than two weeks later, you attended and spoke at the Koch brothers summit in California. This was a betrayal of everything the UNCF stands for."

The UNCF announced the $25 million donation from Koch Industries Inc. and the Charles Koch Foundation last month. The money will go toward a scholarship program, loan assistance and providing general support to historically black colleges and universities.

Lomax spoke at a summit hosted by the Koch Brothers last month, The Nation reported. Through early June, the Koch-backed group Americans for Prosperity had spent at least $44 million on congressional races since August, according to a person familiar with the total.

In a statement, Lomax said he was "saddened" by AFSCME's decision, but insisted that it would not "distract us from our mission of helping thousands of African American students achieve their dream of a college degree and the economic benefits that come with it."

Lomax also defended the group's decision to accept money from the Kochs.

"UNCF has over 100,000 donors with a wide range of views, but they all have one thing in common: They believe in helping young students of color realize their dreams of a college education. For over 70 years we have never had a litmus test and we have asked all Americans to support our cause," he said.

Saunders said AFSCME would continue to "work directly with historically black and other colleges and universities, faculty members, student organizations, and other allies to make internship, scholarship and job opportunities available to students of color."