Michelle Kichline, a former Tredyffrin Township supervisor, will be Chester County's newest commissioner.

Chester County judges selected Kichline on Tuesday to replace Ryan Costello, who is scheduled to be sworn in as Sixth District congressman on Jan. 6.

Costello, the board chairman, will formally resign Monday. Kichline and the judges will determine when she will join the board. She will finish Costello's term, which expires in early 2016.

Since the outgoing commissioner is a Republican, replacement candidates also had to be Republicans, according to Pennsylvania law.

After Kichline joins the board, she and Commissioners Terence Farrell and Kathi Cozzone will vote to determine the chairman.

Chester County commissioners are paid an annual salary of $80,702, according to the county. The chairman gets $81,702 annually.

Kichline will be eligible to run in November's election, when county residents will vote to fill three commissioner spots. Farrell, a Republican, and Cozzone, a Democrat, will be up for reelection.

Brian McGinnis, chair of the Chester County Democratic Committee, said the newest commissioner should not be able to run as an incumbent in November's election. He cited county judges' decision in 2006 to fill State Sen. Andy Dinniman's vacant seat on the board with someone who would not run in the following election.

"Incumbency, and its electoral advantages, should be earned by candidates at the ballot box, not in closed-door meetings in the courthouse," McGinnis said in a statement.

The 14 judges of County Court interviewed three women and three men Tuesday morning at the Chester County Justice Center. They announced their choice that morning.

mbond@philly.com

610-313-8207 @MichaelleBond