Former South Dakota Gov. Mike Rounds, a target of state and national conservatives, now officially has a hurdle in his path to the Republican Senate nomination.

Larry Rhoden, the state Senate majority whip, told the Sioux Falls Argus Leader that he intends to announce a Senate bid on Wednesday. He’s the first to step forward with a GOP primary challenge to Rounds, whose eight-year tenure in the governor’s office left activists on the right unhappy.

Rhoden told CQ Roll Call in March that he was considering running, though conservatives at that time were still awaiting word from GOP Rep. Kristi Noem. She has since declined to run, leaving activists still in search of an alternative.

Given his high name ID and fundraising head start, Rounds remains the early favorite in this top pickup opportunity for Republicans. Democratic Sen. Tim Johnson’s retirement left open a vulnerable seat, and potential Democratic candidates like former Rep. Stephanie Herseth Sandlin and U.S. Attorney Brendan Johnson declined to run.

For now, Democrats’ only candidate is Rick Weiland, a former House candidate and top aide to ex-Sen. Tom Daschle, D-S.D. The party no doubt welcomed the news of Rhoden’s candidacy — its hopes in this GOP-leaning state may rest on a costly GOP primary and weakened Republican nominee.

The race is rated Lean Republican by Rothenberg Political Report/Roll Call.