Sen. Ben Nelson, the most vulnerable Senator up for re-election in 2012, may get a gift in a divided GOP primary, as top-tier Republicans line up to oppose him.

Nelson is the only Democrat in Nebraska’s Congressional delegation, and Roll Call Politics rates his re-election race as a Tossup. Former Nebraska Sen. Bob Kerrey, who chaired the Democratic Senatorial Campaign Committee through two election cycles in the late 1990s, said a competitive Republican primary probably benefits the Democrat.

“I would say eight times out of 10, a competitive primary will be helpful to the opposite party,” he told Roll Call on Friday, although one exception is that candidates in a competitive primary get free media and better name recognition because of the contest.

State Attorney General Jon Bruning and state Treasurer Don Stenberg are strongly leaning toward running, and activist Pat Flynn and state Sen. Deb Fischer have said they’re considering it, too. A Public Policy Polling survey taken in late January showed Bruning led the primary field, getting 47 percent to Stenberg’s 19 percent.

The attorney general has already begun to draw distinctions with Nelson. This week he released a online ad and a website criticizing Nelson’s work for his deal, known as the “Cornhusker Kickback,” on the health care reform bill in 2009. The video features a guy who would “do anything for a special deal.”

“And when Ben Nelson’s deciding vote was cast, even this guy felt the shame of the cornhusker kickback,” an announcer says. “At that moment, his life changed forever. This guy finally realized the consequences of a kickback and vowed to make the obvious choice from now on.”

Bruning, who was re-elected attorney general in 2010 and is part of a lawsuit against the health care law, will undoubtedly try to make a big issue out of Nelson’s vote for the health care bill. Nelson campaign manager Paul Johnson said there’s no telling what issues Nebraska voters will prioritize when it comes to elections that are more than a year away.

“It’s hard to foresee in a year what the environment will be like and what we’ll be talking about,” he said.

Both Bruning and Stenberg have experience in Nebraska Senate races. Bruning kicked off a campaign during the 2008 cycle but ultimately deferred to now-Sen. Mike Johanns (R). Because of that race, he already had money in a federal campaign account.

Stenberg, a former attorney general, has run for Senate three times before. In 1996 he lost the Republican primary to Chuck Hagel, who went on to win but didn’t run for re-election in 2008. In 2000 he won the Republican primary but narrowly lost to Nelson in the general election to replace Kerrey, and in 2006 he again lost the primary, this time to businessman Pete Ricketts, now a Republican National committeeman for the state.

Nelson hasn’t announced whether he will run for re-election, but all signs point to him running. He had $1.5 million in his campaign account at the end of 2010, and he has begun hiring campaign staff. Johnson managed Kerrey and Nelson’s previous campaigns.

“We’re hiring staff. We’re raising money. We’re doing the various elements of putting a campaign together,” Johnson told Roll Call on Friday. “He always takes his time making a final decision, but unless there’s some unforeseen circumstance that causes a change, he’s definitely running.”

Kerrey said it’s likely that if Nelson were to decide not to run, there would be a competitive primary on the Democratic side, too, but he hoped that wouldn’t be the case.

“I’ve talked to him and told him that if he decides to run, I’ll do whatever I can to help him. It’d be a terrible loss for Nebraska if he decides not to run,” Kerrey said.