Oddly enough, he is SO easy to manipulate, that he is almost impossible to manipulate. Every time Kelly, Ryan or McConnell think they have him under control and walking the party line, a butterfly catches his attention and he goes flying off in a weird direction. Take today's twitter post, where the republicans were on the verge of quietly slipping through a piece of legislation that was well known to make the American people suspicious and uncomfortable, when Trump suddenly blows up the whole issue by apparently attacking his own parties legislation because he completely misunderstood what it was. It passed anyway, but it turned out to be the opposite of subtle. Trump isn't really control of his own actions, he just blows around aimlessly in any direction that catches his attention.
I wouldn't be surprised to see Republicans start jumping on the Trump is mentally unfit bandwagon here soon. The best solution they could hope for is to claim that his mental health deteriorated AFTER he won the president, allowing them to remove him in favor of Pence. They could use the confusion to attempt to shutter the Russia investigation, and reorient their image around the economy. It might not be enough, but it is the best shot they have right now.