Coburn announced last night that he is resigning effective the end of the current session in December. Two years remain on Coburn's term.

Under state law, a vacancy such as the one occurring because of Coburn's resignation requires the governor to declare a special election to fill out the remainder of the term, with the election to run concurrently with the regular election.

The election dates will include a June 24 primary, a potential runoff election on Aug. 26, and a general election on Nov. 4.

Unlike in some states, Oklahoma law does not allow for the appointment of a replacement member of Congress, except in a few unusual, short-term circumstances. No appointment is expected in this case.

Coburn, who is undergoing treatment for a reoccurrence of prostate cancer, said Thursday that his decision is not related to his health.

"This decision isn't about my health, my prognosis or even my hopes and desires," he said.

Coburn served three terms in the U.S. House of Representatives, from 1995-2001, then came out of political retirement to win election to the Senate in 2004. He was re-elected in 2010 and said he would not serve past the end of his second term in 2016.

A fierce critic of government spending and waste and something of a contrarian even in his own party, Coburn became somewhat more conciliatory in his second term.

Oklahoma's other U.S. senator, Jim Inhofe, also a Republican, is up for re-election this year.