Rep. Russ Carnahan (D-Mo.) filed Tuesday to challenge Rep. Lacy Clay (D-Mo.) in a primary.

Carnahan’s 3rd district was decimated by redistricting last year, leaving him to choose between running against Clay in his St. Louis-based 1st district, in the open but Republican-leaning 2nd district, or for another office.

With the filing deadline approaching, Carnahan announced he will wage an uphill battle against Clay in a Democratic-leaning district.

“These are the maps we have today,” Carnahan told the St. Louis Post-Dispatch. “I made it clear over a year ago that I’m running for reelection regardless of how the maps turn out.”

The new district is more than two-thirds of Clay’s current district and less than one-third of Carnahan’s. The other parts of Carnahan’s district were divvied up between the new 2nd district, where former Republican National Committee co-chairwoman Ann Wagne r is favored to succeed Senate candidate Rep. Todd Akin (R-Mo.), and the new 3rd district, where Rep. Blaine Leutkemeyer (R-Mo.) is running for reelection.

The state lost one district due to lack of growth over the last decade, and with Carnahan’s seat getting axed, Leutkemeyer’s 9th district will be re-numbered as the 3rd.

When it was drawn, Carnahan reportedly lashed out at his colleagues — including Clay — for throwing him under the bus. The new map was supported by Clay and the state’s other Democratic congressman, Rep. Emanuel Cleaver.

The Carnahan-Clay matchup is just the latest between sitting members of Congress. Redistricting has now pitted a dozen incumbents against one another.

The first will take place next week, when Reps. Marcy Kaptur (D-Ohio) and Dennis Kucinich (D-Ohio) face each other on Super Tuesday.