Rep. Sam Graves will not run against Sen. Claire McCaskill (D-Mo.) in 2012, the Missouri Republican announced Thursday, saying it was “an agonizing decision.”

“However, I also believe that I can have a greater impact on federal policy in the next six years as a chairman in the House,” the House Small Business chairman said in a statement. “I am the first chairman in the history of the sixth Congressional district and there is much I still want to accomplish in Washington.”

The GOP field still has the potential to be crowded. Former state Treasurer Sarah Steelman and former Congressional candidate Ed Martin are already in the race for the Republican nomination. Rep. Jo Ann Emerson and former state GOP Chairwoman Ann Wagner, who just lost a bid to become chairwoman of the Republican National Committee, have also been mentioned as potential opponents for McCaskill.

McCaskill is considered one of the most vulnerable Senators up for re-election in 2012. Roll Call Politics rates this race a Tossup.