Election results
As of 1 June 2020, there are 359,293 registered voters in Chester County.

Democratic: 149,175 (41.52%)
Republican: 148,377 (41.30%)
Other Political parties and no affiliation: 61,741 (17.18%)
Chester County has historically been reliably Republican at the county level; traditionally, it was the most conservative county in the Philadelphia area.
In recent elections, however, it has been trending Democratic, though not as overwhelmingly as the rest of the Philadelphia suburbs.
In 2000 Al Gore lost it by almost 10 percent but in 2004 George W. Bush defeated John Kerry by a much smaller margin of only 4.5 percent.
Bob Casey, Jr. carried it by 10% when he unseated incumbent Republican US Senator Rick Santorum in 2006.
In 2008, Chester County sided with the rest of Pennsylvania and voted for Barack Obama by a much larger margin of 9%, making him the first Democrat to carry it in a Presidential election since 1964.
But, in 2009, with a smaller turnout, Republican candidates swept all county-row offices, winning with an average margin of 20%.
In 2012 the county voted for Republican candidate Mitt Romney, by a very small margin of about 500 votes.

In 2016, despite Pennsylvania voting for a Republican presidential candidate for the first time since 1988, Chester County voted more Democratic than in 2012, with Hillary Clinton leading Donald Trump by over 25,000 votes or 9.4 percentage points; a 4.8 percentage point swing from 2012.
The only two statewide winners in 2016 to carry Chester County were U.S. Senator Pat Toomey (R) and Pennsylvania State Treasurer Joe Torsella (D).
Republican candidates John Brown and John Rafferty carried Chester County, though both lost their races for Auditor General and Attorney General, respectively (Rafferty, a State Senator whose district includes northern Chester County, carried the county by a slim margin of 50 votes)
Democrats have made gains in Chester County state legislative seats in recent elections.
Democrat Andy Dinniman picked up the 19th Senate District in May 2006 in the special election to replace the late Robert Thompson.
Democrat Barbara McIlvaine Smith picked up the open 156th House district in November 2006, winning by 28 votes and tipping the State-House majority to the Democrats.
This was the first time that a Democrat had served part of Chester County as State Representative since Jim Gerlach (who represented much of Chester County for 12 years in Congress) unseated Sam Morris in 1990.
In 2008, two more open House seats in the county went Democratic—to Tom Houghton in the 13th and Paul Drucker in the 157th.
In 2010, however, Chester County swung back to the GOP, with Republicans Dan Truitt (who defeated McIlvaine Smith), Warren Kampf (who defeated Drucker), and John Lawrence (who defeated Houghton) all elected to the State House.

On November 8, 2017, Democrats made historic inroads in Chester County by winning their first County Row Office seats since 1799, picking up 4 row office seats.
On November 5, 2019, Democrats swept countywide row office seat elections and took a majority on the Board of Commissioners, for a first time in county history.

Presidential elections results
Election results
As of 1 June 2020, there are 359,293 registered voters in Chester County.
Democratic: 149,175 (41.52%)
Republican: 148,377 (41.30%)
Other Political parties and no affiliation: 61,741 (17.18%)
Chester County has historically been reliably Republican at the county level; traditionally, it was the most conservative county in the Philadelphia area. In recent elections, however, it has been trending Democratic, though not as overwhelmingly as the rest of the Philadelphia suburbs. In 2000 Al Gore lost it by almost 10 percent but in 2004 George W. Bush defeated John Kerry by a much smaller margin of only 4.5 percent. Bob Casey, Jr. carried it by 10% when he unseated incumbent Republican US Senator Rick Santorum in 2006. In 2008, Chester County sided with the rest of Pennsylvania and voted for Barack Obama by a much larger margin of 9%, making him the first Democrat to carry it in a Presidential election since 1964. But, in 2009, with a smaller turnout, Republican candidates swept all county-row offices, winning with an average margin of 20%. In 2012 the county voted for Republican candidate Mitt Romney, by a very small margin of about 500 votes.
In 2016, despite Pennsylvania voting for a Republican presidential candidate for the first time since 1988, Chester County voted more Democratic than in 2012, with Hillary Clinton leading Donald Trump by over 25,000 votes or 9.4 percentage points; a 4.8 percentage point swing from 2012. The only two statewide winners in 2016 to carry Chester County were U.S. Senator Pat Toomey (R) and Pennsylvania State Treasurer Joe Torsella (D). Republican candidates John Brown and John Rafferty carried Chester County, though both lost their races for Auditor General and Attorney General, respectively (Rafferty, a State Senator whose district includes northern Chester County, carried the county by a slim margin of 50 votes)
Democrats have made gains in Chester County state legislative seats in recent elections. Democrat Andy Dinniman picked up the 19th Senate District in May 2006 in the special election to replace the late Robert Thompson. Democrat Barbara McIlvaine Smith picked up the open 156th House district in November 2006, winning by 28 votes and tipping the State-House majority to the Democrats. This was the first time that a Democrat had served part of Chester County as State Representative since Jim Gerlach (who represented much of Chester County for 12 years in Congress) unseated Sam Morris in 1990. In 2008, two more open House seats in the county went Democratic—to Tom Houghton in the 13th and Paul Drucker in the 157th. In 2010, however, Chester County swung back to the GOP, with Republicans Dan Truitt (who defeated McIlvaine Smith), Warren Kampf (who defeated Drucker), and John Lawrence (who defeated Houghton) all elected to the State House.
On November 8, 2017, Democrats made historic inroads in Chester County by winning their first County Row Office seats since 1799, picking up 4 row office seats. On November 5, 2019, Democrats swept countywide row office seat elections and took a majority on the Board of Commissioners, for a first time in county history.
Presidential elections results
Chester County, Pennsylvania