Missouri State Auditor Tom Schweich (R) has opted against running for Senate against Sen. Claire McCaskill (D-Mo.), putting to bed the hopes of some GOP donors who encouraged him to join the crowded primary field.

"I just spoke to Tom Schweich, and he just announced he is not going to go forward in this race," Jon Brunner, one of the other GOP candidates in the primary, told The Hill. "Now it's back down to three candidates."

Prominent members of the state party had been recruiting Schweich, a former chief of staff to two U.N. ambassadors, to enter the race in early March, hinting that the current field was too weak to be able to reliably unseat McCaskill. Schweich filed the paperwork indicating an interest in the seat, and 18 top Missouri Republicans penned a letter to him urging him to formally enter the race.

Schweich's decision is welcome news for the rest of the GOP field, which includes Brunner, Rep. Todd Akin (Mo.) and former state Treasurer Sarah Steelman. But Democrats are unlikely to forget the fact that Republicans were dissatisfied with their slate of candidates — and that Schweich took a pass at challenging McCaskill — as they defend McCaskill against the eventual GOP nominee.