They surely threw a lot of numbers at him and pretended they stopped this many spies, that many terrorist, bla bla bla.
To bad, there is a Title III which can legally do these things.
To bad most of these numbers are speculative in nature.
He probably knew all along he was going to do this anyway.
First pretend to be conservative, then cave in when you hear the secret briefings and pretend like that changed your mind.
“I have to do this, for our national security.”
It’s a flip-flop intended to make him look reasonable, like some convincing argument changed his mind... I’m sure the bureaucracy/establishment will reward him.