When House Speaker Paul D. Ryan announced Wednesday that he would not seek re-election in November, it left Republicans with another open seat to defend in a midterm cycle that has already led to dozens of retirements.

But even before Mr. Ryan’s surprise announcement, several candidates had emerged to challenge him in the race for Wisconsin’s First Congressional District, a traditionally conservative area in the southeastern corner of the state. Barack Obama carried the district in 2008, but lost it in 2012. In 2016, Donald J. Trump won it by more than 10 points.

The filing deadline to run for the seat is June 1, leaving potential candidates with plenty of time to organize a campaign ahead of the Aug. 14 primaries. Here is a look at some of the candidates already running.