Rounds raises $600K in second quarter

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 JohnsonTim JohnsonFormer GOP senator endorses Clinton after Orlando shooting Housing groups argue Freddie Mac's loss should spur finance reform On Wall Street, Dem shake-up puts party at crossroads 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.

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