"But, what can they do. Once he has sworn the oath, he's President"
He is, and then it is all going to hit him - the economic nightmare, the seemingly insoluble foreign policy problems, the difficulties that will surround the implementation of his 'America First' policies.....and so on and so on.
He will discover that people stop smiling and cheering quite so much when a President doesn't deliver on the promises he made (on the record) to them when he wanted their votes.
He'll discover that simply saying you think global warming is a hoax isn't good enough when you are confronted with the realities of it. He'll be dismayed to find out that Putin just wants to manipulate him, after all, and that the Peoples Republic of China is far too powerful, both economically and militarily, to be pushed around by someone like him.
There are going to be massive learning curves in every direction, stretching as far as the eye can see and the trouble is, a lot of his senior advisers don't know much more about running an administration than he does.
Ronald Reagan was a political giant by comparison, and he had enough of a job getting to grips with things - if it hadn't been for his clever wife he would have failed dismally. Trump has a wife who speaks five languages, and is on record as saying she thinks her husband should behave more like a President, and give up his Twitter account right now. Perhaps she'll prove to be an asset - we'll see.