See this story. The district has been represented by Democrats for several decades.
In Pennsylvania, party meetings choose nominees in special elections; there are no primaries. The Libertarian and Green Parties were able to place a nominee on the ballot with no petition. In special elections, unlike regular elections, parties that polled a certain share of the vote in the last statewide election are on the ballot automatically. Both the Libertarian and Green Parties met that vote test in November 2020. They each needed to have a statewide nominee who polled more than 2% of the highest vote-getting statewide winner’s vote. Although the Green Party wasn’t on the Pennsylvania ballot in November 2020 for president, it was on for the other statewide offices.