Aaron Brown of the Minnesota Brown blog had me hooked when he began his report on the April 14 DFL convention's nonendorsement for the open Eighth Congressional District seat by invoking a little Minnesota political history.

"Forty-four years ago," Brown recalled, "DFL delegates … met in Grand Rapids for a convention that lasted 30 ballots. They endorsed Iron Range state Sen. Tony Perpich. Perpich would, however, lose the primary to the candidate he beat at that convention, Jim Oberstar. Oberstar would go on to serve 36 years in Congress, creating the largely false narrative that this district is easy to understand."

I was schooled in that narrative as a green reporter. It told that while DFLers in CD8 were prone to fierce intraparty battles, they unfailingly delivered DFL victories in November. When Perpich and Oberstar went to war in 1974, the DFL primary was tantamount to a general election.

By contrast, DFL Rep. Rick Nolan's impending retirement this year is giving Republicans one of their best chances in the nation to pick up a congressional seat — and a DFL primary fight might improve the odds for Republican candidate and St. Louis County Board Member Pete Stauber.

Brown's blog inspired some then-and-now reverie — and that steered me to a fine source of political illumination, state demographer Susan Brower. Her data confirmed my suspicion: There's more to the political drift from blue to red in CD8 than admiration for the steel tariffs and Twitter charms of President Donald Trump.

Here's some of what Brower shared:

• A district that many Minnesotans think of as dominated by Duluth and the Iron Range has seen its demographic center of gravity shift to the south and west. The metro area has sprawled northward, making Chisago County more than triple in population between 1970 and 2010, from 17,500 to 54,000, and Isanti County more than double, from 16,500 to 38,000. Kanabec, Crow Wing and Hubbard county populations also doubled in 40 years.

• By contrast, numbers are down in the city of Duluth (86,000 today, down from 100,000 in 1970). St. Louis County, encompassing both Duluth and much of the Iron Range, is home to 20,000 fewer people in 2010 than in 1970. Neighboring Lake and Koochiching counties have also experienced major declines.

The changing timber and taconite industries account for a good deal of that change. St. Louis County Commissioner and former state Rep. Tom Rukavina estimates that 15,000 people worked in the taconite industry in 1974; the comparable number today is about 4,300.

• Income patterns have shifted accordingly. In 1980 — right before a devastating recession on the Iron Range — the highest-income county in CD8 was Lake County. By 2012, it was exurban Chisago, where median incomes were running $7,500 a year above the state average. Its neighbor, Isanti, had median incomes in 2012 that nearly matched the state average of $58,906. Every other county in CD8 registered a household income below the statewide median that year; in 14 counties, median incomes lagged the state average by more than $10,000.

• In 1980, CD8 was home to lots of young families. Today it's a lot grayer. The county with the oldest median age in 1980 was Aitkin, at 36.3 years. In 2010, it was still Aitkin with a median age of 51.7 years, a number that conjures images of retiree residents rimming Big Sandy Lake. Fourteen of the district's 18 counties had median ages older than 40 in 2010. For comparison: The median age in St. Paul in 2010 was 30.9 years.

• In every county, people are better-educated today. In 1970, only one CD8 county — Beltrami, home of Bemidji State University — had a population in which more than 10 percent had earned a bachelor's or graduate degree. Every county in the district had passed that bar by 2012 — but some were racing ahead. In St. Louis and Beltrami counties, both home to public universities, more than 1 of every 4 adult residents had at least a four-year college degree. That was double the share in many other counties.

I mention that educational divide because I've lately read "The New Geography of Jobs," a 2012 book by University of California economist Enrico Moretti. It describes a nation that is "segregating educationally" at a time when educational attainment is the best predictor of whether a region is on the rise or the skids. He calls it "the Great Divergence."

"A country that is made up of regions that differ drastically from one another will end up culturally and politically Balkanized," Moretti warned.

"Balkanized" might already describe Minnesota in general and the Eighth District in particular — richer and rising near the metro area and Duluth; stagnant, poorer and discontented elsewhere.

It adds up to a CD8 that is more conservative than 40 years ago, blogger/radio producer/college instructor Brown said when I told him that his blog had inspired a data dive.

"The Iron Rangers with liberal inclinations have moved away," the Hibbing native told me. "They live in Minneapolis and vote for Keith Ellison now" for the U.S. House. Among those who remain, resentments simmer — against the Twin Cities, against corporate overlords and even against Duluth.

"Rukavina used to call east Duluth Republican. Now it's more DFL than the Range," Brown said. Duluth and the Range "are separated by different economics. They are different kinds of people. They might not understand each other anymore.

"When you put an environmental debate into the mix, that's toxic," he added.

DFLers appear to have been spared a primary brawl over the environmental impact of proposed new copper-nickel mining ventures when Leah Phifer ended her congressional campaign. The former FBI analyst from Two Harbors led on every ballot at the April 14 convention but failed to pass the 60 percent bar for endorsement. Remaining in the race are two more pro-mining candidates, state Rep. Jason Metsa of Virginia and former state Rep. Joe Radinovich of Crosby, as well as former TV anchor Michelle Lee and North Branch Mayor Kirsten Kennedy. .

Neither Metsa nor Radinovich is old enough to remember the hammer-and-tongs DFL primary of 44 years ago, which featured one of the state's first no-holds-barred fights over abortion. Maybe their relative youth (Radinovich is 32, Metsa 37) is a good thing. Given all that's changed in the Eighth District, they shouldn't want to replicate it.

Lori Sturdevant is a Star Tribune editorial writer and columnist. She is at lsturdevant@startribune.com.