Remember, “centrists” are mainly uninformed morons or people with strange views. This story is big enough that it breaks through and penetrates the views of those that don’t follow politics. I think there’s still another 5% that will flip, and then he’s looking at close to 2/3 in favor of impeachment. Hell, this Syria thing may do that.
I’m not going to say that they are “calculating”, but when the penalty for flagrantly ignoring subpoenas and other congressional requests has been zero to this point, you’ve basically conditioned the White House to do whatever they want.