Most of the Republican primary campaign for U.S. Senate in Nebraska has been about two candidates: Ben Sasse and Shane Osborn.

Not anymore.

In this March 11, 2014 file photo, Republican Senate hopeful, Omaha banker Sid Dinsdale, participates in a debate in Omaha. (AP Photo/Nati Harnik)

Allies of Sasse, who is running with the tea party mantle, have ramped up attacks in recent days against Sid Dinsdale, a wealthy bank executive who has emerged as a real threat during the final week of the campaign. At the least, he's a threat to peel votes away from Sasse; at most, he's a threat to win.

On Thursday, the Club for Growth hit the airwaves with an attack ad casting Dinsdale as too liberal. The Club supports Sasse. Earlier this week, the Madison Action Fund, another group backing Sasse, released a radio ad hitting Dinsdale. Last week, conservative blogger Erick Erickson, another Sasse supporter, wrote a piece highlighting Dinsdale's relatives' ties to groups that support abortion rights.

The focus on Dinsdale suggests his campaign has become a not-to-be-ignored factor down the stretch. Osborn, the GOP establishment choice, has faded as Sasse has surged ahead. But Dinsdale is the wild card heading toward Tuesday's primary. He loaned his campaign $1 million and is hitting the airwaves hard during the final week of the race.

Sasse's allies have been pounding Osborn. Osborn has been slamming Sasse. The negative war may have opened the door for Dinsdale, who was not regarded as much of a factor throughout most of the race.

The 2014 primary bears some similarities to the 2012 campaign. In that race, a nasty ad war between establishment-favored candidate Jon Bruning and tea party-backed Don Stenberg allowed a third candidate, Deb Fischer, to sneak through and pull out an unlikely win.

Fischer is now a U.S. senator.

"Negative campaigning doesn't work in this election. It didn't work in Deb Fischer's race," Dinsdale campaign manager Beth Kramer told The Fix last week.

But the question is whether Dinsdale can use the three-way competition to his advantage and spring an upset a la Fischer. It will be more difficult because Sasse has put himself in a better position that Stenberg did. Sarah Palin, who came in late for Fischer, backs Sasse.

Lots of other leading national tea party groups and figures have lined up squarely behind Sasse, so a loss would be a big black eye for the movement -- especially in a cycle without many promising opportunities elsewhere on the map.

Sens. Ted Cruz (R-Tex.) and Mike Lee (R-Utah), and Palin have stumped with Sasse. He's got a robust TV presence down the stretch. He's put himself in a position to win.

But in a state where unpredictable outcomes are not uncommon, Sasse is no sure bet, thanks to Dinsdale.