Senate Budget Chairman Don Nickles (R) announced Tuesday he will not seek re-election next year, setting off what is likely to be a competitive race for Oklahoma’s open seat.

Nickles was scheduled to make his announcement at 11 a.m. local time in Oklahoma, several sources said. His decision to forgo a fifth term ends a 24-year career that spanned four presidents and began in 1980 when he was elected to the Senate after serving only two years in the Oklahoma state Senate.

Nickles becomes the fifth Senator to announce his retirement this year. Democratic Sens. John Edwards (N.C.), Fritz Hollings (S.C.), Zell Miller (Ga.) and GOP Sen. Peter Fitzgerald (Ill.) have all already announced they will leave the chamber at the end of this Congress. Democrats are waiting to see if Sens. John Breaux (D-La.), and Bob Graham (D-Fla.) also decide to retire.

Oklahoma Rep. Brad Carson and state Attorney General Drew Edmondson are possible Democratic contenders for the open seat, while Oklahoma Rep. Ernest Istook and Oklahoma City Mayor Kirk Humphreys are two Republicans who have expressed interest in a campaign.