Donald Trump

He said Mr Trump's attempt to discredit a poll before it has even taken place was "unprecedented" for a US presidential candidate.

Also "unprecedented", said Mr Obama, was the Republican candidate's "flattery" of Russia's president.

Mr Trump is facing sinking poll numbers and accusations of sexual assault.

The businessman-turned-politician has claimed the 8 November election will be "absolutely rigged" for Democratic candidate Hillary Clinton.

Mrs Trump called those accusations "lies" and saying Mr Trump was "egged on" into "boy talk" during a 2005 tape in which he made lewd comments about women.

"I believe my husband. I believe my husband," she said in an interview with CNN's Anderson Cooper on Monday.

"This was all organized from the opposition. And with the details ... did they ever check the background of these women? They don't have any facts."

She also said she hadn't heard her husband use that kind of language before.

His comments appear to contradict his running mate Mike Pence, who told NBC earlier Mr Trump would "absolutely" accept the election result, despite media "bias".

Mr Trump's adviser Rudy Giuliani has also accused Democrats of "cheating".

Polls suggest Mr Trump is losing ground in some of the key battleground states.

Meanwhile Hillary Clinton's running mate Tim Kaine has blasted Mr Trump's election-rigging claims as "scare tactics".

He also suggested the presidential race was looking like a "rigged election".

The comments came in the wake of the publication of a recording in which he made obscene remarks, which sparked a string of sexual assault claims.

Polls suggest Mr Trump is losing ground in some of the key battleground states.

Speaking at a rally in New Hampshire, Mr Trump said Mrs Clinton had been "all pumped up" at the beginning of the last debate but could "barely reach her car" at the end.

"The other day I'm standing at my podium and she walks in front of me, right? She walks in front of me and when she walked in front of me. Believe me, I wasn't impressed, but she walks in front of me," he said at a campaign rally in Greensboro, North Carolina.

It was not clear what he meant when he said he "wasn't impressed" and a message left with the Trump campaign was not immediately returned. But Trump has commented on women's appearances in recent days, particularly those who have made sexual assault allegations against him.

In the May 7, 1998 appearance, obtained and reviewed by CNN's KFile, Trump also said "women are worse than men, they're more sexually aggressive than men," adding, "If they're married they're even worse."

Trump also spoke at length about his Miss Universe contest, telling Stern and co-host Robin Quivers, "we don't base it on talent, we don't base it on brains."

In the hours of crude and misogynistic sex talk Trump engaged in when he appeared on The Howard Stern Show, one of the most common topics of discussion were famous women.

Trump and Stern would often rate, rank, and describe in sexual terms female celebrities. In appearances reviewed by CNN's KFile, Trump also told Stern he would pay not to receive oral sex from Rosie O'Donnell and teased a recently-deceased Anna Nicole Smith for her large lips.

A Trump campaign spokesperson did not respond to a request for comment on this story.

Former Apprentice contestant Summer Zervos said the businessman forced himself on her at a Los Angeles hotel and began "thrusting his genitals".

Kristin Anderson told the Washington Post he reached up her skirt and groped her in a New York club in the 1990s.

The Republican nominee branded the allegations as "lies and smears".

Ms Anderson, now 46, said the property mogul touched her through her underwear at a Manhattan nightspot when she was a waitress trying to make it as a model.

She said she was "very grossed out and weirded out".

The Republican presidential nominee said the women making the allegations were "horrible liars" and the media was colluding with rival Hillary Clinton.

Several women say he groped them or forcibly kissed them.

First Lady Michelle Obama earlier said Mr Trump's boasts about groping women were "shocking and demeaning".

"Leaders should meet a basic standard of human decency," she said during a campaign appearance in New Hampshire.

Two women told the New York Times that the Republican presidential runner groped or kissed them.

A reporter for People magazine wrote a first person account describing being forcibly kissed by the property tycoon.