He ran for the Senate in 2004, entering the race relatively late, but he still easily beat the Republican field and then defeated former U.S. Rep. Brad Carson in the general election.

Coburn had limited himself to two terms in the Senate, so he would have been retiring in 2016.

Editor's note: You can read the entire text of Sen. Coburn's statement below.

“Serving as Oklahoma’s senator has been, and continues to be, one of the great privileges and blessings of my life. But, after much prayer and consideration, I have decided that I will leave my Senate seat at the end of this Congress.

“Carolyn and I have been touched by the encouragement we’ve received from people across the state regarding my latest battle against cancer. But this decision isn’t about my health, my prognosis or even my hopes and desires. My commitment to the people of Oklahoma has always been that I would serve no more than two terms. Our founders saw public service and politics as a calling rather than a career. That’s how I saw it when I first ran for office in 1994, and that’s how I still see it today. I believe it’s important to live under the laws I helped write, and even those I fought hard to block.

“As a citizen legislator, I am first and foremost a citizen who cares deeply about the kind of country we leave our children and grandchildren. As I have traveled across Oklahoma and our nation these past nine years, I have yet to meet a parent or grandparent who wouldn’t do anything within their power to secure the future for the next generation. That’s why I initially ran for office in 1994 and re-entered politics in 2004. I’m encouraged there are thousands of Americans with real-world experience and good judgment who feel just like I do. As dysfunctional as Washington is these days, change is still possible when ‘We the People’ get engaged, run for office themselves or make their voices heard. After all, how else could a country doctor from Muskogee with no political experience make it to Washington?