The GOP field against vulnerable Missouri Sen. Claire McCaskill might not be set yet.

Show-Me State Auditor Tom Schweich is considering entering the race, according to a Missouri Republican familiar with his thinking. Schweich supporters have even filed paperwork with the IRS to begin the process of setting up an organization that could support his campaign. That news was first tweeted by Greg Giroux of Bloomberg News.Businessman John Brunner, former state Treasurer Sarah Steelman and Rep. Todd Akin (Mo.) are the current contenders for the GOP nomination. Some Missouri Republicans have groused for months that their field could be stronger against McCaskill, whom they see as a weak incumbent.

“I don’t know if he’s fully in yet, but he is getting a lot of encouragement from prominent Republican donors,” a state Republican source said.

Schweich, an attorney, served as chief of staff to three U.S. ambassadors to the United Nations and as a principal deputy assistant secretary of state.

The filing deadline for Missouri is March 27. Roll Call rates the Missouri Senate race as Tossup.