Minnesota Democrats said it felt as if their state was a true battleground for the first time in decades.

“Over the last 25 years, I’ve never seen an investment as robust and deep this early by a Republican presidential candidate,” said Ken Martin, the chairman of the Minnesota Democratic-Farmer-Labor Party. Mr. Martin said it was clear to him that “Trump and the Republicans believe they can win and are doing everything to make it so.”

Few states in the Midwest exemplify Mr. Trump’s strengths, such as popularity in rural areas, as well as his weaknesses, such as his shaky standing in suburban areas, the way Minnesota does. Which group turns out in bigger numbers could determine who wins the state on Election Day.

If Mr. Trump succeeds in flipping Minnesota’s 10 electoral votes, it will probably mean that he has secured himself another four years in the White House. No Democratic candidate can carve a path to victory without Minnesota, and Trump officials are eager to put Democrats on defense there. One campaign official compared the dynamic to Republicans having to spend resources to defend a Republican stronghold like Indiana.