Donald Trump savaged US media on Thursday for reporting "outright lies" that he groped and forcibly kissed women, as First Lady Michelle Obama blasted the Republican nominee in an emotional tirade.

Just 26 days before the American electorate chooses a successor to her husband on Nov 8, a visibly angry first lady delivered a fiery takedown of the real estate mogul and his "disgraceful" behavior.

"Now is the time for all of us to stand up and say, enough is enough," Obama told a rally for Democratic nominee Hillary Clinton in New Hampshire. "This has got to stop right now."