Is "Donald Trump finally dropped the ball" the right expression?

One popular expression I have heard in animes is that he made a blunder of a lifetime. While I couldn't find any reference for this idiom, here's a link to dictionary meaning.

According to the freedictionary:

A mistake typically caused by ignorance or carelessness.

http://www.thefreedictionary.com/blunder

Maybe this would suit your use?

shoot oneself in the foot

Foolishly harm one's own cause, as in He really shot himself in the foot, telling the interviewer all about the others who were applying for the job he wanted. This colloquial term alludes to an accidental shooting as opposed to a deliberate one done so as to avoid military service.

[shoot oneself in the foot. (n.d.) The American Heritage Dictionary of Idioms by Christine Ammer. (2003, 1997). Retrieved December 14 2015 from http://idioms.thefreedictionary.com/shoot+oneself+in+the+foot .]

If you shoot yourself in the foot, it's not likely to be more than a once in a lifetime mistake, although doing it twice is a remote possibility.