In the Missouri governor's race, the National Rifle Association (NRA) could have endorsed a former Navy SEAL who fired an assault rifle and Gatling-style military machine gun in an ad, and is Republican. Instead, it has thrown its support behind a Democrat.

The political arm of the group chose Missouri Attorney General Chris Koster over his Republican challenger Eric Greitens, the NRA's Political Victory Fund announced Tuesday. While Mr. Greitens, a political newcomer, received a strong rating from the NRA, Mr. Koster has a long record of "fighting to preserve the Second Amendment," said the group.

Koster, a Republican until 2007, is out of step with other Missouri Democrats on gun issues. But he is also part of a dying breed of Democratic politicians across the country that the Washington-based NRA supports. Like many other issue-based organizations, the NRA has become more partisan over the last five years, reported The Trace, a news site that focuses on guns, in May.

“The plummet in the NRA support for Democrats highlights the group’s increasingly partisan posture, which is at odds with its past attempts to avoid party allegiances. It also highlights bigger political shifts,” wrote Dan Friedman for The Trace.

An analysis by The Trace and the New York Daily News found the primary spending tactic of the NRA has become sponsoring attack ads against Democrats. The NRA spent at least $33.4 million on attack ads from 2010 to 2016, mostly in an effort to defeat Democrats, they found.

But Koster won over the group with his 17-year voting record, most notably his public endorsement of Missouri Proposition B in 1999, which, if it had passed, would have required law enforcement to issue concealed weapons permits to eligible adults. Koster was the only Missouri prosecutor to publicly endorse the right-to-carry referendum, according to NRA-PVF.

Koster is also out of step with other Missouri Democrats in supporting a state Senate bill the governor vetoed this summer. The bill ends the need for a permit to carry a concealed weapon and reduces the penalty for carrying a firearm into a building where it is not allowed from a felony to a misdemeanor, according to the Kansas City Star.

Koster is also the first Democratic running for statewide office to be endorsed by the Missouri Farm Bureau, reports the Star.

The NRA’s endorsement of Koster is in line with the group’s long-established “incumbent-friendly” policy. If two candidates are equally supportive of gun rights, the incumbent typically “gets the nod,” Ben Pershing wrote for The Washington Post in 2010.

Jennifer Baker, a spokeswoman for the NRA, indicated this is the case with Koster in an interview with the St. Louis Post-Dispatch. Ms. Baker said Greitens, the former Navy SEAL, has proven he supports the Second Amendment, but he hasn’t held public office, so he has no public record.

The Greitens campaign downplayed the endorsement.

"We're not gonna win the career politician insider endorsement contest," Greitens campaign manager Austin Chambers told Politico. "But no one is stronger than Eric on Second Amendment issues and no one will fight harder to grow jobs and clean up the same old politics as usual holding Missouri back.”

Still, the NRA endorsement highlights the options for pro-gun Missouri voters – options that are becoming rarer across the country in this political climate.

Times have changed since the NRA endorsed 58 incumbent House Democrats in 2010, as The Washington Post reported. “Although the NRA’s agenda usually aligns with the GOP, having clout with Democrats has also been useful,” wrote the Post's Mr. Pershing at the time, pointing to House Democrats’ pressure on then-Attorney General Eric Holder not to reinstate an assault weapons ban.

“It it hadn’t been for those 60 House Democrats ... things would have turned out very differently,” Andrew Arulanandam, an NRA spokesman, told the Post.

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Since then, however, the NRA's bipartisan support has waned. In May, The Trace found the NRA had contributed to only one House Democrat’s campaign – a reflection of how partisan many Washington organizations have become.

“Most organizations that have an ideological bent have become partisan organizations as the parties have become more homogenous,” David Wasserman, a congressional reporter for the Cook Political Report, told The Trace. “It’s not exclusive to the NRA. We see it with Planned Parenthood and the Human Rights Campaign. They have become Democratic organizations almost exclusively.”