Hagan has struggled to answer for her support for the health care law, and in a clear sign of Democratic concern, the Senate Majority PAC, which backs Democratic candidates, bought $750,000 of television air time in December to counter Republican attacks against her. The group spent hundreds of thousands of dollars more for Hagan earlier in the year.
North Carolina's primary is May 6 and if no candidate gets 40 percent of the vote, a runoff is set for July 15.
Collins, the Senate Republicans' campaign director, maintained that competition in the primaries will make the party's eventual nominees stronger for the general election.
Republicans see a potential to expand the field from the top tier races to contests in Michigan and Minnesota. Iowa seemed like a prime opportunity for Republicans after five-term Democratic Sen. Tom Harkin announced he would not seek another term.
Democrats rallied around four-term Rep. Bruce Braley. But on the GOP side, there are no fewer than seven candidates seeking the nomination in Iowa, including conservative radio host Sam Clovis, state Sen. Joni Ernst, former energy company CEO Mark Jacobs and former U.S. Attorney Matt Whitaker. There is talk that even more will enter the race.
Iowa's June 3 primary has a 35 percent threshold. If no candidate gets that much, the nomination would be decided at a party convention where the most conservative members typically nominate a harder-right candidate.