Nebraska Attorney General Jon Bruning (R) has been the favorite from the start to succeed retiring Sen. Ben Nelson (D-Neb.), but his hold on that mantle has always been tenuous.

First, state Treasurer Don Stenberg nabbed the backing of the influential Club for Growth and Sen. Jim DeMint’s (R-S.C.) Senate Conservatives Fund. Then, former senator Bob Kerrey got in the race and gave Democrats a fighting chance.

FILE - In this Feb. 11, 2012 file photo, former Alaska Gov. Sarah Palin, the GOP candidate for vice-president in 2008, speaks in Washington. (AP Photo/J. Scott Applewhite, File)

But with just five days until the GOP primary, neither Stenberg nor Kerrey is looking like Bruning’s biggest obstacle. Instead, the until-now-dark-horse candidate in the race, state Sen. Deb Fischer, has asserted herself and — according to politicos in Nebraska — has a fighting chance to usurp Bruning on Tuesday.

Fischer, 61, is an eight-year state senator and product of a ranching family whose father served in Gov. Kay Orr’s (R) Cabinet (more on her bio here). Through late April, she had raised less than $400,000 for her campaign, while Bruning and Stenberg have either spent millions (Bruning) or had millions spent by outside groups on their behalf (Stenberg).

Fischer made perhaps her biggest splash of the campaign Wednesday, announcing the backing of Sarah and Todd Palin. A few days prior, her campaign released a poll showing her within a few points of Bruning, trailing 30 percent to 26 percent, with Stenberg at 18 percent.

One Republican consultant who supports Stenberg says the efforts to boost him have largely failed, and he has slipped into third place, but that the anti-Bruning campaign spearheaded by the Club for Growth has successfully brought down Bruning’s numbers, forced Bruning and Stenberg to battle, and Fischer has shot the gap.

The Club and the Senate Conservatives Fund have combined to spend more than $2 million on the race, with the Club opposing Bruning and the SCF propping up Stenberg.

After previously running ads against Stenberg, Bruning’s campaign is now up with an ad targeting both of his top opponents — a sign of Fischer’s momentum.

“...Deb Fischer is picking up the pieces that are falling from Bruning’s campaign,” said the Stenberg supporter.

Bruning’s campaign is dubious that Fischer has a shot. It argues that Fischer’s polling has over-sampled the Lincoln media market, which is the only market where Fischer’s meagerly funded campaign is up with ads on broadcast TV.

The Fischer poll is the only poll to show anybody within double digits of Bruning, and there has been a dearth of polling in the race.

Bruning has suffered from being the establishment candidate and his more liberal positions from his college days have long dogged him. But he has also taken care to cover his right flank. To date, he has earned the backing of the Tea Party Express, Mike Huckabee and Rick Santorum.

“Jon Bruning’s fight against Obamacare, and record of cutting waste and fraud has created strong momentum for his campaign,” said campaign adviser Brooks Kochvar. “Despite millions in spending against him, he continues to garner ... endorsements from the state’s three largest papers, the chamber of commerce, and Rick Santorum all in the last two weeks.”

But his brash candidacy has proved an obstacle, and despite a big early lead, his hold on the nomination has never been firm. The Omaha World-Herald has run a series of tough stories for the attorney general focusing on a land deal with executives whose business he was tasked with overseeing how Bruning got rich while in public office. Then the paper’s editorial board issued one of the most half-hearted endorsements in endorsement history.

The Palins appeared to allude to the wealth issue in their endorsement, saying of Fischer: “We admire your conservative principles and know that you will not go to Washington to amass great wealth or power.”

Orr, who is chairing Fischer’s campaign and played a major role in securing the Palins’ endorsement through a series of e-mails to Todd Palin, said the endorsement could make the difference next week.

“I think she’s going to win,” Orr said. “I think this is just exactly what she needed to bring additional attention to her campaign.”

Even Stenberg’s supporters are suggesting that Fischer is a worthy compromise, even if their candidate doesn’t win.

They also note, though, that there are lots of undecided voters (about one-quarter in Fischer’s poll) and that Stenberg remains broadly popular in the state GOP thanks to several previous statewide campaigns — including for Nelson’s seat.

“Voters are leaving Jon Bruning and going to Don Stenberg and Deb Fischer, who are more conservative,” said Matt Hoskins, spokesman for the SCF. “Bruning had a 30-point lead, but now it’s a competitive, three-way race.”

That’s true, and though it’s not wholly surprising that Bruning faces some uncertainty in next week’s primary, the candidate who could beat him is.