Yamiche Alcindor:

What we know is that Washington was really, really shocked when Nikki Haley made this announcement.

And it's because our "NewsHour" producers here studied the list of the last 30 years of U.N. ambassadors. And in almost every single case, when that person resigned in the middle of a president's term, it's because they were taking another job in the administration. That is not the case with Nikki Haley.

I spoke with someone who's close to Nikki Haley who also helped get elected in South Carolina. That person says she's very politically ambitious, in a good way, and that she would know when to leave. And in this case, she's leaving when the administration's having a good couple of weeks. They had Brett Kavanaugh confirmed. The trade deal went through.

You also have the economy doing well. So Nikki Haley is kind of leaving on this high. There's, of course, this idea that she could run in 2020, but Nikki Haley was very firm today. She said she's not running in 2020 and she's actually going to help President Trump campaign.

So she's throwing her support behind the president. But that's pretty much what people are talking about today.