Rep. Russ Carnahan (D-Mo.) has filed to run in the Democratic primary against Rep. Lacy Clay (D-Mo.) in a heavily-Democratic, St. Louis-based district.

The decision comes after months of waiting on Carnahan to decide whether he would challenge Clay or choose to run in a Republican-leaning district that includes much of his old territory.

Carnahan’s move is a loss for national Democrats: he would have likely been their strongest option to run in the new suburban district.

{mosads}The race is a continuation of a fight between the two that started with redistricting, when Missouri lost one of its congressional seats. Carnahan’s district was dismantled by Republicans in the state Legislature, but after Missouri’s Democratic governor, Jay Nixon, vetoed the maps, the final votes to override the veto came from Clay allies in the statehouse.

After it became clear Clay was happy to see Carnahan lose his seat, Carnahan swore at him on the House floor, according to reports. Since then he has stayed silent on where he would run, but many predicted he would take on Clay — if he wins he’ll have a safely Democratic district for a decade rather than a perennial swing district.

Still, the race will be an uphill battle for Carnahan, a centrist white Democrat facing a more liberal
African-American congressman in a heavily black district.

Carnahan has a small lead in fundraising, with $449,000 in the bank at the end of the year to Clay’s $387,000.

Democratic Congressional Campaign Committee Chairman Steve Israel (D-N.Y.) told reporters Tuesday morning that he hadn’t yet spoken to Carnahan about his decision.

“Russ called me yesterday,” Israel said. “I haven’t been able to get back to him.”

The race could also heighten tensions between the Congressional Black Caucus and the fiscally centrist New Democrat Coalition: Clay is a CBC member, while Carnahan is a New Democrat.

And it’s not the only member-vs.-member race with this dynamic. Rep. Gary Peters (D-Mich.), a New Democrat, is running against CBC member Rep. Hansen Clarke (D-Mich.), and former New Democrat Coalition member and former Rep. Debbie Halvorson (D-Ill.) is challenging Rep. Jesse Jackson Jr. (D-Ill.), a CBC member, in their primary.

Peters and Clarke have stayed relatively civil, but Jackson Jr. and Halvorson have had a highly contentious primary. With the bad blood already displayed between Carnahan and Clay, this primary is likely to fall in the latter category.

The race is also a matchup between scions of Missouri’s two most powerful Democratic families. Carnahan’s father Mel was governor and head of the party, his mother was a senator and his sister is the state’s secretary of state. Clay’s father, Rep. William Lacy Clay Sr. (D-Mo.), was a longtime congressman and co-founder of the CBC.

A big part of the reason for these New Democrat-Congressional Black Caucus matchups is redistricting. Many African American-majority districts lost population in the last decade, forcing them to expand into the suburban territory many New Democrats represent. Centrist Democrats of all stripes have been targeted by Republicans in redistricting as well, since they often represent swing territory where small changes in the lines can have a big impact.

—This story was last updated at 12:35 p.m.