For the second time in two weeks, super PACs will play a major role in determining the outcome of a U.S. Senate primary contest. Republican Jon Bruning, Nebraska’s attorney general, was expected to win in a cakewalk for the seat, soon to be vacated by retiring Sen. Ben Nelson, a Democrat. Instead, two underfunded insurgent candidates — Don Stenberg and Deb Fischer — are giving him a run for his money, thanks in large part to a handful of outside groups. Bruning has the fundraising advantage, having raised more than $3.6 million for his campaign. Stenberg has raised about $750,000, while Fischer has raised less than $440,000 for the race, including $35,000 of her own money. But heading into today’s primary, conservative outside groups have spent more than $2 million on advertising, according to Federal Election Commission records, with nearly $1 million going toward ads attacking Bruning. The ads appear to have been effective — Bruning’s numbers have slipped, according to recent polls.