Could Minnesota ever become a battleground state?

Despite its long history as a Democratic stronghold in presidential elections, is it possible Minnesota could someday join neighboring Iowa among the ranks of battleground states?

Pundits have long wondered when Minnesota might turn from a blue to a purple state. Various U.S. electoral maps, like FiveThirtyEight's 2016 Election Forecast and those from other sources, have highlighted Minnesota as a state to watch in 2016.

But Minnesota and its 10 electoral votes have been a reliable stronghold for Democratic presidential candidates, who have won the state all but three times since 1932.

The Republican outliers to win the state were Dwight D. Eisenhower in the 1950s and Richard Nixon in 1972. Famously, Minnesota was the only state lost by Ronald Reagan to Democratic challenger and native son Walter Mondale in 1984.

Before a long run of Democratic wins, Minnesota was also one of the few states won by Teddy Roosevelt during a third-party presidential run as the candidate for his Progressive Party (aka the Bull Moose Party) in 1912.

Minnesota is traditionally bluer than bordering red states North and South Dakota, sometimes-tossup Wisconsin and perennial battleground Iowa. But could that be changing?

One way to answer that question is to examine the Cook Partisan Voter Index (CPVI or PVI), which judges how Democratic or Republican a state or district is by comparing a party's voting percentage with the national average from the past two presidential elections.

The PVI has long been considered a gold standard for tracking electoral leanings over time. It's calculated by finding the average Republican and Democratic shares of the two-party vote in a state or district spanning the previous two elections. Those numbers are compared to the national popular vote percentages for each party's candidate in those years. This way, it's possible to judge how much more Republican or Democratic a state or congressional district votes compared with the country as a whole.

This year, the Cook Political Report has listed Minnesota as a likely win for Democrat Hillary Clinton, with a PVI score of D+2 derived from 2008 and 2012 election numbers -- meaning the state leans two points more Democratic than the national average.

However, Minnesota's PVI has gradually decreased over the years, falling from its status as a Democratic stronghold in the 1980s to a slight lean during the administrations of presidents George W. Bush (R) and Barack Obama (D).

Clinton has a six-point edge over Republican candidate Donald Trump, according to the most recent Star Tribune Minnesota Poll. So while the North Star State is likely to remain in the blue column in 2016, within a few election cycles it could become more competitive if voting trends continue to push the PVI down.

Data Drop is a weekly feature that uses data analysis and visualizations to explain, surprise, inform and entertain readers on topics relevant to Minnesotans. Do you have an idea you'd like us to explore? Contact MaryJo Webster