Despite an increasing lead over businessman Donald Trump in head-to-head matchups, presumptive Democratic presidential candidate Hillary Clinton is struggling to connect in some reliably blue states.

Washington Post authors Philip Rucker and John Wagner report that there's concern among Clinton supporters who believe Trump might do well in three Rust Belt states: Michigan, Pennsylvania and Wisconsin.

"In Michigan, Pennsylvania and, to a lesser extent, Wisconsin, an affinity for Trump's message of economic populism and nationalism has surprised many Democrats," Rucker and Wagner wrote. "These are big, industrial states they have carried for the past three decades — and where Clinton, so far, has not fully focused."

In Pennsylvania, Clinton and Trump are in a statistical tie, according to RealClearPolitics polling averages. In Michigan, Trump closed the gap between the two candidates considerably once he became the GOP nominee, and now trails Clinton by just 4 points (though the last poll was taken at the end of May).

Clinton lost to Vermont Sen. Bernie Sanders in Michigan and Wisconsin (but won Pennsylvania), so her team now believes he can be a help in the general election, given that his message on trade deals is closer to Trump's than Clinton's message has been in the past.

Even in swing states where Clinton's campaign is more confident, the polls aren't much better. Clinton leads Trump by 3.5 points in Florida and nearly 3 points in Ohio.

Still, Team Clinton doesn't appear to believe there's a real chance she could lose these states to Trump. Referring to Trump's low favorability rating, Democratic pollster Mark Mellman told the Post that Trump was "looking at fantasy land if he thinks he can put places like California and Michigan into play."

"Clinton aides express a steely confidence about all three states. They think the Democratic National Convention in Philadelphia next month will generate positive attention across Pennsylvania," wrote Rucker and Wagner.

"In Michigan, they are doubtful that Trump could outperform Mitt Romney, who was born here and whose father was a popular governor. Romney lost to Obama by nearly 10 percentage points in 2012. And in Wisconsin, Clinton allies are heartened by the reservations many prominent conservatives there have voiced about Trump."

But the closeness of the polls has prompted pro-Clinton super PAC Priorities USA to make a $10.5 million ad buy in Pennsylvania between July and November. The Democratic Party has also been beefing up field staff in many tough states, including Pennsylvania.

In the end, Clinton will have to convince voters that she understands their unique challenges better than Trump does. Trump's populist approach to working-class Americans may not be resonating in polls now, but there's no telling what will happen between now and election day. Clinton is going to have to start paying attention to these states if she doesn't want a potential surprise loss.

Ashe Schow is a commentary writer for the Washington Examiner.