Former Sen. Jim Talent (R) has decided against challenging Sen. Claire McCaskillClaire Conner McCaskillThe American economy is stronger than ever six months after tax cuts The Hill's Morning Report — Sponsored by PhRMA — Immigration drama grips Washington Conservative group calls for ethics probe into McCaskill’s use of private plane MORE (D-Mo.) and will make his decision public soon, a leading Missouri GOP operative confirmed to The Ballot Box.

For months, observers and party officials believed Talent was contemplating a rematch with McCaskill, who defeated him by fewer than 50,000 votes in 2006. But he's now poised to sit out the 2012 Senate race, sources said.

His decision to pass on making a run means that former Treasurer Sarah Steelman remains the only declared Republican candidate in the field, although that could change quickly in the wake of Talent's decision.

Rep. Sam GravesSamuel (Sam) Bruce GravesHouse passes bipartisan water infrastructure bill Lawmakers left with more questions than answers on Trump infrastructure plan Five obstacles to Trump's infrastructure ambitions MORE (R-Mo.) recently told The Ballot Box that he's contemplating a Senate run and that Talent's plans would influence his decision.

"Jim's a friend of mine; Sarah's a friend of mine. That absolutely factors into it," he said

Talent, who has worked as a lobbyist, is a distinguished fellow at The Heritage Foundation in Washington, D.C. He hadn't been in touch with Missouri GOP activists in months, another Missouri Republican operative said, which seemed to indicate his lack of interest.

Other potential candidates include Lt. Gov. Peter Kinder and Ann Wagner, the former state party chairwoman who recently ran for the chairmanship of the Republican National Committee.

The challenge for Missouri Republicans is to avoid a bruising Senate primary and channel their candidates into the right races. Almost all the state's top constitutional offices — governor, lieutenant governor, attorney general, treasurer — are up in 2012.