Clinton’s plans include an early, aggressive attempt to defend Pennsylvania, Wisconsin and Michigan — reflecting a growing recognition inside her campaign of the threat that Trump’s unconventional bid for president may pose in unexpected places, particularly in economically struggling states that have been hit hard by global free-trade agreements.

Joel Benenson, Clinton’s chief pollster and senior strategist, acknowledged that Trump’s popularity, particularly among white, working-class voters, could make states in the country’s industrial midsection more competitive than they have been in recent elections.

“There is no state where they can put us on defense that we don’t already treat as a battleground,” Benenson said. He added: “The key here is to really protect the territory we have to protect, then play offense.”

Clinton performed poorly against Sen. Bernie Sanders of Vermont in Democratic primaries in this part of the country — partly because of her past support for free-trade agreements and partly because Sanders’s promises to focus on economic issues and income inequality resonated with voters. Those factors could work against her with Trump, who has criticized her positions on trade and has also found deep appeal among the working class.