FILE - In this Dec. 19, 2013 file photo, Sen. Tom Coburn, R-Okla., left, walks with Sen. Richard Burr, R-N.C., on the way to a procedural vote on a comprehensive defense bill as the Senate’s legislative year nears to a close, at the Capitol in Washington. Coburn's announcement late last week that he will forego his final two years in office means Oklahoma will not only have two US Senate races this year, it is expected to trigger a series of dominoes on the GOP side as politicians on the Sooner State's deep Republican bench mull a run. (AP Photo/J. Scott Applewhite, File)