Joe Biden leads President Donald Trump by 13 percentage points among all Pennsylvania registered voters, according to a Monmouth University Poll released Wednesday. But most who were surveyed also believe they live among an untold number of “secret Trump voters” who could ultimately swing the November election.

More than half of those surveyed said they believe their communities are populated by people who support Trump but have not told anyone. Voters were split on whom they expect to win Pennsylvania this year, with 54% saying they were surprised that Trump prevailed four years ago.

Biden, the presumptive Democratic nominee, is polling particularly well in 10 swing counties where the margins were tightest in the 2016 election, the survey found. In those counties, which include the Philadelphia suburbs and Biden’s hometown of Scranton, Biden holds a 54% to 35% lead among registered voters.

Pollster Patrick Murray said it’s the voters in those swing counties, not hidden Trump supporters, who are most likely to decide the 2020 election in the state.

“These are the kind of historical Democratic voters, the old steel-mill voters, who abandoned the Democratic Party in 2016,” he said. “It seems like Biden has significant strength there.”

In August 2016, a Monmouth Poll had Hillary Clinton ahead of Trump by eight percentage points in Pennsylvania. Clinton won Philadelphia decisively, but Pennsylvania’s rural and rust belt communities delivered enough votes for Trump to eke out a 44,000-vote win and become the first Republican nominee in nearly three decades to capture the state’s Electoral College votes.

In Lackawanna, Luzerne, and Westmoreland Counties, Clinton drew fewer votes than Barack Obama did against Republican John McCain in 2008. And support for Trump was better than expected in some longtime Democratic cities like Wilkes-Barre and Scranton.

Biden’s lead shrinks when run through a model of lower voter turnout, Murray said. And overall in some areas, he’s not much further ahead than Clinton was at about this point in 2016 — meaning those places could still be up for grabs.

Polls and studies have not uncovered evidence that “secret Trump voters” existed in large enough numbers in 2016 to swing the election, Murray said, but some Democratic voters are still scarred by what they saw as Clinton’s unexpected loss.

“People are still trying to make explanations for that surprise,” he said.

Monmouth’s poll was based on phone interviews with 401 registered voters in Pennsylvania. Results for the overall sample are subject to a margin of error of plus or minus 4.9 percentage points.

Of those polled, 53% said they supported Biden, and 40% went with Trump. Biden is ahead with voters who are under 50 years old and over 65, while Trump has an edge with voters between 50 and 64. White voters without college degrees were more likely to support Trump than white college-educated voters and voters of color. More than 40% of voters said they were “more enthusiastic than usual” about the election.

The poll also found that 56% of registered voters say Trump has done a bad job handling the coronavirus pandemic, with 42% approving of his performance. About 70% of voters, meanwhile, said Gov. Tom Wolf has done a good job managing the crisis in the state.