The second-quarter sum is much-improved over his first-quarter $184,000 haul, a total that caused some observers of the race to wonder whether he could launch a credible campaign for the seat.

With Sen. Tim JohnsonTimothy (Tim) Peter JohnsonSenate GOP rejects Trump’s call to go big on gun legislation Court ruling could be game changer for Dems in Nevada Bank lobbyists counting down to Shelby’s exit MORE's (D-S.D.) decision to retire, Republicans have seen a top pickup opportunity open up, and they nabbed their preferred recruit in Rounds.

Democrats' top two recruits opted out of the race, and their only announced contender is two-time House candidate Rick Weiland.

Rounds, however, remains open to a primary challenger, as conservative groups, including the Club for Growth, are opposed to his candidacy.