Wirth served as a Democrat in the U.S. Senate from 1987 to 1993 and was in the U.S. House from 1975 to 1987.
He has run the United Nations Foundation for 15 years, which "links people, ideas and resources with the UN to help address the world's major challenges from climate change and children's health to empowering women and stemming population growth," he wrote in a letter Monday.
Wirth said in the letter he would continue to stay on with the foundation's Board and will continue to focus on climate change and population issues he calls "the foremost problems of our time."
The Foundation was founded by CNN mogul Ted Turner.
Read more of Tim Wirth's story in the Denver Post.
(Copyright 2012 The Denver Post)