With the convention less than 2 weeks away we would normally expect the running mate for the challenging candidate to be announced by now, or imminently. From a politically strategic point of view, it seems like it would have made sense to have done it just before the debate. Of course, Trump hasn’t announced yet, and I believe he will wait until the last possible minute to do so.
The reason is obvious: whomever Trump picks is going to be at the center of attention for at least a few days, and Trump just hates not commanding the spotlight. He can’t avoid it happening, but he just can’t bear the thought. He probably wishes he didn’t have to choose a running mate at all.
It wouldn’t surprise me if he waited until after 4PM ET on Friday July 12 to minimize the news coverage as much as possible, although he may let it out tomorrow since on the national holiday far fewer people will be paying attention to the news.