Michelle Obama has denounced Donald Trump at a rally for Hillary Clinton at a university in New Hampshire, saying his lewd comments about sexually assaulting women "have shaken me to my core".

The First Lady on Thursday described the Republican presidential candidate's comments unearthed in a 2005 video as "shocking" and "demeaning."
"Last week we saw this candidate actually bragging about sexually assaulting women," she said.

"I can't believe that I'm saying that a candidate for President of the United States has bragged about sexually assaulting women. I can't stop thinking about this. It has shaken me to my core in a way that I couldn't have predicted."

"I'd love nothing more to pretend that this isn't happening," the First Lady continued. "This is not something that we can ignore. This is not something we can just sweep under the rug as just another disturbing footnote in a sad election season."

"It doesn't matter what party you belong to -- Democrat, Republican, Independent -- no woman deserves to be treated this way. No one deserves this kind of abuse," she added.

"This wasn't 'locker room banter,'" the term Trump used to describe his words. "This was a powerful individual speaking freely and openly about sexually predatory behaviour."

Later, Obama prompted the crowd to finish her now infamous quote from July's Democratic National Convention.

"Remember this, when they go low, we go (high)," she called out. "Yes we do." The quote has become a motto for Democrats, with Clinton repeating it in last week's debate.

Obama also echoed Clinton's debate speech, saying "America's greatness comes from recognising the innate dignity and worth of all our people ... that this country is big enough to have a place for us all: men and women, folks of every background and walk of life ... and we are always stronger together."

The First Lady encouraged attendees not only to vote, but "step up and start organising," including making calls, going door-to-door, and sharing stories on social media.

"The choice you make November 8th could determine whether we have a president who treats people with respect or not ... a president who thinks women deserve the right to make our own choices about our bodies and our health, or not," she said.