Voter turnout across the Philadelphia region surged Tuesday to levels not seen in at least a decade in municipal elections, with 30% more voters than usual going to the polls in some counties, according to preliminary data.

In the suburban Pennsylvania counties where Democrats picked up historic wins, tens of thousands more ballots were cast than in recent off-year elections.

In Chester County, where Democrats took control of the commissioner board for the first time, nearly 40% of the county’s 351,925 registered voters cast ballots on Tuesday, according to unofficial results. That’s about 30,000 more than the county tallied in 2017 local elections — a 28% increase. The uptick far outpaced the increase in voter registrations over that period of time.

Democrats also swept to power in Bucks and Delaware Counties, and maintained control of the Montgomery County Board of Commissioners.

The number of votes cast in Bucks and Montgomery increased by nearly 20% over recent local elections. Delaware County officials hadn’t released turnout data as of Friday, but unofficial results there showed about 160,000 people voted in the district attorney’s race. That’s means about one-third more voters showed up at county polls Tuesday compared with the 2017 municipal election.

G. Terry Madonna, a pollster at Franklin and Marshall College, attributed the surge in turnout to the nation’s political climate. National tensions have trickled down, stirring more people to vote in election years that traditionally have low turnout, he said.

“What we have now is a motivated electorate,” Madonna said. “I don’t think there’s any doubt that leading up to this presidential election, with a highly polarized electorate, Democrats are overwhelmingly opposing Trump and Republicans are supporting him.”

In Philadelphia, which saw competitive at-large City Council races, 28% of the city’s 1,062,588 registered voters cast ballots — up from 20% two years ago and 25.8% during the last mayoral election in 2015.

The city added about 60,000 people to its voter rolls over the last four years. The number of total votes cast increased by more than 38,000, or 15%, from 2015 to 2019. That may seem small, but some at-large City Council races were decided by narrow margins.

Lisa Deeley, a city commissioner, attributed the increase to “a combination of social media and something in 2016 that made people realize how important their vote is and what’s at stake.”

County-level data on the number of registered Democrats and Republicans who voted wasn’t available. In addition, the election returns will change slightly as absentee and provisional ballots are counted.

Lee Soltysiak, Montgomery County chief operating officer, said he has seen an upward trend in voter participation since 2016.

Soltysiak said he had predicted an increase in voter turnout this year based on the number of absentee ballots requested.

“We’ve used the number of absentee ballot requests as a barometer of voter interest,” he said. “From 2016 to now, we went from 3,500 to 8,500 requests.”

While Democratic voters appeared to show enthusiasm in the election, high turnout was seen in Republican counties as well.

In Westmoreland County in Southwest Pennsylvania, a former Democratic stronghold with nearly 237,000 registered voters, turnout increased from 27% two years ago to 36% Tuesday. The number of votes increased by 18,600, or 28%, even as the number of registered voters fell by about 8,000. Republicans took control of the county board of commissioners there.

In the central part of the state, Republican-dominated Lycoming County saw 38% turnout, on par with 2015 levels but up more than one-third from two years ago.

Staff writer Jonathan Lai contributed to this article.