White House hopeful Hillary Clinton suggested on Monday that rival Donald Trump could be an unpredictable adversary in upcoming presidential debates, saying she is girding for "wacky stuff".

With the first debate scheduled to take place a month from now near New York, Clinton joked about what to expect from her unorthodox opponent.

"You've got to be prepared for wacky stuff. I'm planning on drawing off my experiences from elementary school," she said on late-night talk show Jimmy Kimmel Live.