Go through this list and note who’s on it — and who’s not on it.

Today I’m proud to endorse such a wide and impressive array of Democratic candidates – leaders as diverse, patriotic, and big-hearted as the America they’re running to represent: pic.twitter.com/gWzalQhFas

— Barack Obama (@BarackObama) August 1, 2018

Alexandria Ocasio-Cortez is the most notable omission, but not the only notable one.

Notable omissions from Obama’s 2018 endorsements list:

• Alexandria Ocasio-Cortez (NY)
• Beto O’Rourke (TX)
• Claire McCaskill (MO)
• Heidi Heitkamp (ND)
• Kyrsten Sinema (AZ) https://t.co/A8y5usDXEa

— Frank Luntz (@FrankLuntz) August 1, 2018

At first I thought maybe O was avoiding Senate endorsements altogether right now for whatever weird reason, but no, he endorsed Jacky Rosen in Nevada. That’s probably because, in an age where most purple states are trending red, Nevada is a purple state that’s stayed mostly blue. In 2010, with a big red wave crashing down all over the country, Nevada stuck with Harry Reid over Sharron Angle. Six years later, with Pennsylvania, Michigan, Wisconsin, and Florida breaking for Trump, Nevada went for Hillary by two points. In that same election, Reid’s seat passed to Democrat Catherine Cortez Masto instead of Republican Joe Heck, one of the few disappointments for Republicans nationwide.

That’s what distinguishes Rosen from O’Rourke, Sinema, McCaskill, Heitkamp, and many others on the ballot this fall — in her state, the Obama seal of approval is probably still more of an asset than a liability. How happy do you think Ted Cruz would be to have an Obama endorsement of his opponent to show off in Texas? Obama’s not going to hand him a gift like that.

But Ocasio-Cortez is a different ballgame. She’s from the safest of blue districts, a mortal lock to win her race this fall. Could be that O is friendly with Joe Crowley, the man she upset in the primary, and doesn’t want to step on his toes by endorsing her (particularly when he’s still technically on the ballot this fall). But that can’t be it. Crowley himself has endorsed Ocasio-Cortez and no one thinks he has a chance to win as a minor-party candidate unless something happens to knock her out of the race. Loyalty to Crowley isn’t the reason.

The reason Obama didn’t endorse her, I assume, is that he thinks the “socialist” label would damage Democrats nationally this fall by motivating Republicans to vote. And nothing would elevate that label as quickly or as widely as an official presidential Hopenchange endorsement of Ocasio-Cortez. You can write the headlines yourself. “IT’S OFFICIAL: OBAMA, DEMOCRATS EMBRACE SOCIALISM.” Or “OBAMA CONFIRMS HE WAS A SOCIALIST ALL ALONG.” It’d be one thing if the seat was competitive and she needed a boost from him to win it, but she doesn’t. So what does she, or the party, gain by him giving it? Better to maintain the illusion that “mainstream Democrats” and Democratic socialism are two kinda sorta different things than rashly drop the pretense to the contrary and risk an unpredictable reaction. Especially since, if the climate gets really rough for the GOP this fall, red-state candidates like Cruz, Martha McSally, Kevin Cramer, etc may try to use Ocasio-Cortez in their own races to goose turnout. Do you want this party, now led by avowed socialists, to take over Congress this fall? they’ll say. Then get your asses to the polls. Obama associating himself with Ocasio-Cortez would make that message easier to sell to righties. And O knows it.