Onder is the second high-profile candidate to enter the race to replace U.S. Rep. Blaine Luetkemeyer, who announced he was retiring when his term ends this year. Republican state Sen. Mary Elizabeth Coleman announced her bid for the seat last month.

Luetkemeyer’s exit is expected to trigger a crowded Republican primary for a seat that leans heavily in the GOP’s favor. After squeaking into office by 2.5% in 2008, Luetkemeyer has never received less than 63% of the vote and has averaged 67% over the past five elections.

Among the names being floated include former state Rep. Sara Walsh, former Boone County Clerk Taylor Burks and former state Sen. David Schatz.

The 3rd District runs through all or part of 16 counties, from the Lake of the Ozarks to the Mississippi River. Almost half the population is in eastern and northern St. Charles County, the southern half of Boone County and the western half of Jefferson County.

Onder, a physician who ran for Congress and lost to Luetkemeyer in 2008, said in a statement announcing his candidacy that “Washington DC has never been more broken than it is right now.”

“Throughout my public service, I have stood up to Democrats and my own party establishment to get things done,” he said, “and with our country at such a critical juncture, that is why I want to go to Washington to represent Missouri’s 3rd District.”