Sen. Tim Johnson’s (D-S.D.) path to re-election appears measurably easier this morning, as former Lt. Gov. Steve Kirby (R) removed his name from consideration as a candidate.

“My family and I have been overwhelmed by the outpouring of support and encouragement from many of our South Dakota friends in recent weeks as I have considered a bid for the United States Senate,” Kirby wrote in an e-mail Monday morning.

“Suzie and I want to thank all of our friends who have done so much to support us during this time of decision. I have decided, however, not to seek the 2008 GOP nomination for the United States Senate at this time.”

Kirby, a potential self-funder who has previously held statewide office, was the National Republican Senatorial Committee’s lone top-tier recruit to challenge Johnson this year.

With Kirby out, Republicans are left with two lightweight candidates vying for the GOP Senate nomination — and neither is expected to give Johnson much of a race in November, despite South Dakota’s conservative tilt. State Rep. Joel Dykstra and businessman Sam Kephart are running for the Republicans.

Kirby was seen as dangerous enough to Johnson that the Senator’s campaign and the Democratic Senatorial Campaign Committee attacked the Republican last week, even though he had yet to make a decision on whether to run. Kirby’s decision to remain in private life this cycle leaves Sen. Mary Landrieu (D-La.) as the only legitimate pickup opportunity that Senate Republicans appear to have this cycle.

— David M. Drucker