Wisconsin Rep. Tom Petri will announce Monday he is not seeking re-election this year, according to his office .  

First elected in 1979, the Republican is in his 18th term representing the 6th District, based in Oshkosh and Sheboygan.  

The district is potentially competitive. Mitt Romney carried it with 53 percent in 2012, while President Barack Obama won it narrowly with 49 percent four years earlier.  

Following Petri’s announcement, Republican state Sen. Joe Leibham announced he is mulling a bid for the seat. State Rep. Duey Stroebel has expressed interest in the race, one state Republican said, while state Sen. Glenn Grothman had already announced a primary challenge to Petri.  

State Democrats named Manitowoc Mayor Justin Nickels and Winnebago County Executive Mark Harris as potential candidates.  

Petri, one of the 50 richest members of Congress, is the 24th House member to retire this cycle.  

For a full list of retirements, check out Roll Call’s Casualty List .