The Republican Main Street Partnership PAC — which is supporting a group of 53 moderately conservative members of Congress — will endorse Rep. Steve King's GOP opponent Randy Feenstra for Iowa's deeply conservative 4th district.

Why it matters: The group, which advocates for a sizable, if shrinking part of the Republican caucus, is the first national GOP organization to publicly endorse and financially support King's primary challenger.

The backdrop: King has come under fire from both his Republican and Democratic colleagues on the Hill for racist and xenophobic comments. Some congressional leaders, like House Republican committee chair Liz Cheney (R-Wy.), have called for his resignation.

Last January, GOP leadership stripped King of his committee assignments after he asked in a New York Times interview how terms like "white nationalist" and "white supremacist" became offensive.

In August, King drew condemnation again for defending a ban on abortion with no exceptions by arguing that most of the world's population wouldn't exist without rape or incest.

The state of play: Feenstra, an Iowa state senator, is one of four Republican primary rivals trying to defeat King, who has held onto his seat more than 15 years.

Feenstra raised more than $721,000 in 2019. He out-raised King in at least three quarters of 2019, according to the Sioux City Journal.

King narrowly won his midterm race against Democrat J.D. Scholten in 2018, defeating him by just 3%. Scholten is running again this year.

What they're saying: “As a landowner in Iowa’s 4th District, it’s frustrating to know that nothing is getting done on our behalf in Congress. I know how this system works,” says Doug Ose, RMSP PAC‘s treasurer and a former congressman. “The district deserves representation."

Go deeper: Iowa newspapers urge Steve King to resign over racist remarks