One has to assume there was a backroom push for Trump to endorse Ryan, McCain AND Ayotte. The inclusion of Ayotte, who unlike Ryan didn't actually stab the Tea Party in the back, screams deal. Otherwise, Trump would have been better off saying nothing:
1. A loss by Ryan will probably win the election for Trump. Not only will it confirm the anti-incumbent wave, but it will also reinforce the Narrative. The media will report GOP voters tossed out their own speaker and 2012 VP candidate in a primary, never done before in history. The Narrative of outsider vs insider, reform vs stasis would have been complete.
2. A loss by Ryan will break the back of the GOPe. Ryan was popular with the grassroots for many years because he is a relatively conservative politician. If Ryan can be crushed in a primary, all the GOPe can be crushed in a primary. In the wake of a Ryan loss, President Trump could legitimately threaten to primary any GOPe resistance to his agenda, or the grassroots could do it for him.
3. Now that he has endorsed Ryan, Trump loses some political capital if Ryan loses.
The latest polling shows Ryan will win his race easily, in which case a Trump endorsement is close to worthless. If he was able to trade it for anything, he came out ahead on the deal.
Charlie Rose - From Radar in 2007:
3 hours ago