State Rep. Stace Nelson joins South Dakota Senate Majority Whip Larry Rhoden, physician Annette Bosworth and former Gov. Mike Rounds in the GOP primary.

Nelson will officially announce his campaign on Aug. 18, according to The Argus Leader.

Rounds remains the establishment pick in the race, but he's long faced criticism from conservatives on his record. The Senate Conservatives Fund put him on blast on day one of his campaign, announcing it would be searching for a "conservative alternative" to the candidate.

While Rhoden has framed himself as that alternative, he hasn't yet gained traction with national GOP groups. Nelson is expected to run to Rounds's right as well — the Republican was kicked out of the House GOP caucus because of clashes with party leadership.

Democrats currently have only former congressional candidate Rick Weiland, considered a second-tier option after two of the party's more prominent contenders decided against the race. National Democrats say they're still looking for a candidate to defend the seat.