PHILADELPHIA — The Pennsylvania Democratic Party on Wednesday sued Erie County’s board of elections over mail ballot issues that may have affected between 10,000 and 20,000 voters in the bellwether county known to choose the presidential winner statewide — and chose President Joe Biden in 2020 by only 1,400 votes.

Erie County’s mail ballot issues have been unfolding for weeks.

The county announced earlier this month that about 300 voters received ballots intended for others in addition to their own because of an error by its Ohio-based vendor, ElectionIQ.

Another 1,800 ballots were never mailed by the vendor, the county said, so unless the affected voters reach out to elections officials, they will not receive their mail ballots in time to vote in the Nov. 5 election.

Attorneys for the Democratic Party contend in the suit that the party “has been made aware by its registered party members of numerous instances where a voter requested a mail-in ballot weeks ago but has still not received their ballot.” They also point to the mail ballot returns in Erie County, which trail the statewide return averages. Only 52% of voters who requested a mail ballot returned one as of Oct. 28, attorneys said, or 21,536 of the 40,844 requested. This is 15 percentage points less than the statewide average, where 67% of voters who requested a mail ballot have returned one.

In all three scenarios, the party alleges that the county has yet to take action to ensure that voters can cast their ballots.

The party has asked the court for an injunction to require the county board of elections to release the names of the affected voters, require the board of elections to have extended hours through Nov. 4, and create an observation process for the party to identify voters who received additional mail ballots to confirm their eligibility to vote for the candidates on that ballot, among other requests.

While Republicans and former President Donald Trump’s campaign have already filed several lawsuits in Pennsylvania, this is the first suit the state Democratic Party has made ahead of the election over mail ballots.

An attorney for Erie County’s board of elections could not immediately be reached for comment.

Erie is one of Pennsylvania’s best-known bellwether counties, and residents know the familiar refrain: “where Erie goes, so goes Pennsylvania.” For that reason, it’s a destination for politicians campaigning in the state, and both Vice President Kamala Harris and Trump have made frequent stops to the northwestern part of the state.

Erie was one of three counties to flip from voting for Barack Obama in 2012 to Trump in 2016. It was also one of two, along with Northampton, to flip back to Biden in 2020 — though by the thinnest of margins, about 1,400 votes.

Wedged along Lake Erie between Cleveland and Buffalo, the county is a reflection of the industrial Midwest. It has the mid-sized city of Erie, politically balanced suburbs, and conservative rural areas with cornfields, dairy farms, and sprawling vineyards used for Welch’s grape juice and Rust Belt wineries. Its geography makes it a microcosm of Pennsylvania. The margins in most statewide elections are within a tenth of a percentage point of how Erie voted.

Despite its Democratic, labor union roots, Erie County saw a 21,000-vote swing to Trump compared with the previous election, a huge factor in a state decided by just 44,000 votes. Ever since, political analysts, campaign operatives, and reporters trying to tap into the Pennsylvania voter psyche have watched Erie for signs of whether Trump-like candidates can hold white, working-class voters across Midwestern swing states.

In addition to its impact on the presidential election, Erie is expected to be decisive on downballot races, such as row offices and two state legislative races that could decide who controls each chamber.