On Saturday, 215 Republican delegates will vote for their nominee to serve the remaining months of former Congressman Tim Murphy's term.
Murphy, who championed anti-abortion legislation, resigned last month following reports he asked a woman with whom he had an extramarital affair to get an abortion.
Republican nominees vying for the open seat are state Representatives Jason Ortitay and Rick Saccone, state Senators Guy Reschenthaler and Kim Ward, and George Karpacs, member of the Pennsylvania College Access Program board.
Pennsylvania's 18th District is historically heavily Republican. In the 2016 election, Donald Trump carried the District by 20 points. But Democrats are mobilizing for the possibility to cinch the seat that includes parts of Allegheny, Greene, Washington and Westmoreland counties.
Left candidates include 23-year Navy Veteran Pam Iovino, Westmoreland County Commissioner Gina Cerilli and former federal prosecutor Conor Lamb. Democratic leaders will select their nominee Nov. 19.
The special election will be held March 13 for a position that will last for eight months. An election to fill the next full term for the 18th Congressional District seat will take place in November 2018.
Vice chairman of the Republican Committee of Allegheny County Dave Majernik told 90.5 WESA in October that whoever wins the special election will have an advantage in November.
"Normally, whoever gets selected in this process will be the one who runs again in 2018 for the full term, probably without challenge."