William Brangham:

Well, Paul Manafort now has to cooperate fully and completely with Mueller's team. He has to answer whatever questions they want, turn over documents. There's nothing that seems to be off-limits in their questioning.

What Paul Manafort can tell Robert Mueller is still the $64,000 question. We just don't know. I mean, if you think about what Mueller's mandate is, what did Russia do vis-a-vis our elections and what role did the Trump campaign, if any, play in that?

Paul Manafort was a central character in the Trump campaign. He was at this infamous Trump Tower meeting. But, again, we just don't know what he has to offer.

That said, this certainly has to make the president quite nervous. I mean, the president all along has been very complimentary to Paul Manafort, saying how glad he was that he didn't break under pressure, comparing him to Michael Cohen.

So now that the reverse has happened today, it's got to be unsettling for the White House.