On October 5, Pennsylvania U.S. House member Tim Murphy, a Republican representing the 18th district, said he will resign from the U.S. House effective October 21. This means a special election is likely, probably in December 2017 or January 2018. In Pennsylvania, in special elections, qualified parties choose their nominees by convention.
The Libertarian Party and the Green Party have limited status as qualified parties in Pennsylvania. Although they must petition for their nominees in regular elections, they don’t need any petition in special elections. The 18th district is in southwest Pennsylvania. Ever since the district was created in 2011, there have been no minor party or independent candidates in that district.