Given that it's the season for ghoulish metaphors, I'm emboldened to report that a ghost is haunting northern Minnesota politics this year.

I can almost hear Jim Oberstar's booming laughter upon learning that I'm referring to him.

The late Oberstar's unexpected 2010 defeat after 36 years in the U.S. House has been much on both Republican and DFL minds in both his beloved Eighth District in northeastern Minnesota and the adjacent Seventh, which runs along nearly the entire western border of the state.

Republicans point to Oberstar's loss to Republican newcomer Chip Cravaack four years ago as evidence that they can take both districts from their current DFL occupants, U.S. Reps. Collin Peterson of the Seventh and Rick Nolan of the Eighth. Why?

• It's again a mid-presidential-term election. That typically means reduced turnout — and that typically favors Republicans.

• Demographic changes are working in the GOP's favor, particularly in northeastern Minnesota.

Loss of population on the traditionally DFL Iron Range combines with metro exurbia's seepage into the southern portion of the Eighth District — where Cravaack's Tea Party holds sway — to change the Eighth District's partisan tint from Democratic blue to purple. In 2012, President Obama won the Eighth with a squeaky 51.5 percent of the vote.

The Seventh tilts right, save for populist precincts in the Red River Valley that hew to their Farmer-Labor heritage. Republican presidential candidate Mitt Romney carried the district by a 10-point margin in 2012.

• Oberstar's defeat shows that seniority does not provide security. A perverse aspect of congressional service is that while advancing tenure enhances one's stature in Washington, it also brings vulnerability at home to charges of being "out of touch" or having "gone Washington."

Peterson is long-serving — though Oberstar's 18 terms make Peterson's 12 seem unremarkable. Nolan is the boomerang congressman, sent back to Washington in 2012 after a 32-year hiatus. He represented the old Sixth District from 1974 to 1980.

• Policy leadership doesn't buy protection, either. Oberstar was chair of the powerful House Transportation and Infrastructure Committee when he was defeated — a position that allowed him to do much good for Minnesota, including securing fast and full federal funding for the replacement Interstate 35W bridge in 2007.

Peterson is reminding voters that he's the ranking minority member and former chair of the House Agriculture Committee, and that he's been a chief architect of the last several federal farm bills. One might think that would matter in ag-dominated western Minnesota. But roads, bridges, trains and airports should have mattered in the Eighth in 2010, too.

• Mostly unspoken — at least for attribution — is a chronological parallel. Oberstar was 76 when he was defeated. An avid bicyclist with a soaring intellect, he was as seemingly fit as septuagenarians come. But he was still the subject of whispers about the toll of advancing years.

Peterson and Nolan both blew out 70 candles on their last birthdays. By comparison, state Rep. Torrey Westrom, Peterson's Republican challenger, is 41; Nolan's GOP challenger Stewart Mills, scion of the Mills Fleet Farm family, is 42.

I wish I could run all that by Oberstar for a witty retort. He stunned all who knew him when he didn't wake up on May 3 this year. The glowing tributes that ensued reminded Minnesotans of the asset they lost when voters replaced him with a greenhorn who held the seat for only one term.

But Peterson and Nolan are quite available. Both paid recent visits to the Star Tribune. When I invoked the name Jim Oberstar, both were ready to try to convince me that his fate would not be theirs.

"Jim got painted with some things that took hold — that he didn't have a house [in the district], he didn't come back much, and when he did, he stayed in hotels," Peterson said. "I don't have that problem."

But Peterson's use of his personal airplane has been an issue. The National Republican Congressional Committee used Twin Cities television ads — a weapon seldom used in Seventh District contests — to blast Peterson for collecting $280,000 in airplane mileage reimbursement. "He's been there so long, he's forgotten it's our money," the ad scolded.

What the ad doesn't say is that the $280,000 total covers 24 years.

"That's cheap! This is being reimbursed to do my job," Peterson said, noting that all members of Congress receive mileage reimbursement for travel. "I go by myself. Anybody else in my district would have staff driving them. They'd be staying overnight, and have hotel rooms and meals." His mode of transportation is a bargain for taxpayers, he says.

Nolan, too, cited Oberstar's inattention to his home fire. "Jim was in the district once or twice in the year he got beat," Nolan said. "There was not a lazy bone in Jim's body. He was out around the country, helping other people get elected."

By contrast, Nolan said, "I'm all over the district. I've been home every weekend. That's one of the lessons I took from Jim's experience."

Another such lesson responds to Reason No. 1 for GOP optimism. If the midterm turnout lag again works to Republican advantage, this time it won't be because Nolan & Co. didn't employ every trick they know to turn out DFL voters.

No big-name Democrats visited Minnesota in 2010. There was no U.S. Senate race on the ballot that year. Until the campaign's final days, no one in Washington believed that Oberstar was in trouble. He evidently didn't believe it himself.

Last week, as Vice President Joe Biden stumped for Nolan in Hibbing, I wondered if the trip was his act of penance for not coming to Jim's aid four years ago.

Lori Sturdevant, an editorial writer and columnist, is at lsturdevant@startribune.com.