“We’re just getting started,” said Club for Growth spokesman Barney Keller, who said the group will soon begin training its fire on Democrats. “Our group has already had an impact on what the composition of Congress is going to look like next year. That’s our whole goal is to have an impact, to improve the gene pool in Congress.”

While Democrats welcomed the unexpected chance to compete for the Indiana seat, many are increasingly worried about the threat posed in the fall by well-funded conservative groups.

Leading Democratic lawmakers, including Senate Majority Leader Harry M. Reid (Nev.) and House Minority Leader Nancy Pelosi (Calif.), have stepped up their fundraising efforts in recent months on behalf of liberal-leaning super PACs, which can raise unlimited money but have fallen far short of matching their conservative opponents.