What to look at is that it's never that blue. In a sense it's voted Democrat as long as the bluest, but I'd only classify it as light blue or likely Democrat alongside CT, MI, IL, NV, WI, NM, WA, OR, NJ, and ME. It's a teaser too, just close enough for Republicans to say "this is our year in MN." At 52-53% of the popular vote, Republicans could carry Minnesota.
Like in 2004 when Bush won 53% of the vote and carried Minnesota?
Oh wait.
I could see Minnesota going GOP if the Republican got 57% of the national vote and there wasn't a Minnesotan on the Democratic ticket. Or any other popular statewide DFLer.
The GOP will have won the presidency loooong before that... so the GOP pumping money into MN is really a case of fools chasing fools gold.
And I'll reiterate it... again. If you look at the actual policies and statistics, you'll find Minnesota is actually quite liberal in practice. They accept the DFL as the de facto ruling party and only use the GOP as a punishment when the DFL overreaches.