US President Barack Obama sharply criticised Donald Trump over his repeated assertions that the 8 November election is rigged against him.
He told the Republican presidential candidate to "stop whining and go try to make his case to get votes."
With opinion polls showing him falling further back in recent days against his White House rival, Democrat Hillary Clinton, Mr Trump has intensified his allegations, although numerous studies have shown voter fraud in US elections is rare.
At a campaign rally in Colorado, Mr Trump continued his attack, saying: "The press has created a rigged system and poisoned the minds" of voters.
But he also used his speech to unveil a new initiative if elected president: a constitutional amendment imposing term limits for members of Congress.
Mr Obama was asked about Mr Trump's assertions at a joint news conference in the White House Rose Garden following meetings with Italian Prime Minister Matteo Renzi.
He responded with a strong attack on the Republican candidate, noting that US elections are run and monitored by local officials, who may well be appointed by Republican governors of states, and saying that cases of significant voter fraud were not to be found in US elections.
Mr Obama said there was "no serious" person who would suggest it was possible to rig the elections, adding, "I'd invite Mr Trump to stop whining and go try to make his case to get votes."
Mr Trump has raised the possibility for months of illegal activities that could tarnish the November election result, and he has urged his supporters to show up at polling locations on election day.
He has sharpened his allegations in recent days, even as Republican lawyers called his assertions unfounded.
Mr Trump's vice presidential running mate, Mike Pence, took a softer tone than Mr Trump during a campaign appearance in North Carolina, saying he was "confident" that the integrity of the election will be ensured.
But Mr Pence noted that "in recent years we've had instances, proven instances of voter fraud."
The New York real estate developer currently trails Mrs Clinton by seven points in national opinion polls, according to an average of polls compiled by Real Clear Politics.
Throughout the election campaign, Mrs Clinton has argued that Mr Trump, with his divisive rhetoric and attacks against Muslims and other minorities, is not qualified to lead the United States.
Mr Obama echoed that theme, arguing that Mr Trump's focus on rigged elections "doesn't really show the kind of leadership and toughness that you want to have in a president. You start whining before the game is even over?"
The Democratic president added, "If whenever things are going badly for you and you lose you start blaming somebody else, then you don't have what it takes to be in this job."
Over the last few weeks, Mr Trump has been embroiled in one controversy after another, eclipsing debate around the sort of policy topics such as taxes, trade, the economy and national security that normally dominate the final push in presidential campaigns.
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Melania Trump stands by her man despite 'boy talk'
Melania Trump has shrugged off her husband's "boy talk", despite a number of sexual assault accusations against Mr Trump and recordings of him boasting that he uses his fame to grope and kiss women.
Just as her billionaire husband has, the Slovenian-born ex-model criticised the media for unfair treatment of the Republican White House nominee, who has recently lost ground in the polls to his Democratic rival.
Ms Trump turned the blame on the host of the show that aired the tape of Mr Trump boasting that as a star he can grab women "by the p****".
The comments were made to Billy Bush on Access Hollywood in 2005. Mr Bush, a nephew of former president George H.W. Bush, is leaving NBC, the network announced last night.
"I said to my husband that, you know, the language is inappropriate, it's not acceptable," Ms Trump said in the sit-down interview. "And I was surprised because that is not the man that I know."
"I wonder if they even knew that the mic was on because they were kind of boy talk and he was led on, like, egged on from the host to say dirty and bad stuff."
She said that she had never heard her husband use such vulgarity.
"I don't know that person that would talk that way and that he would say that kind of stuff," she said, before writing if off as "boy talk".
"The boys, the way they talk when they grow up and they want to sometimes show each other, oh, this and that and talking about the girls," she went on.
"But yes, I was surprised of course."
Mr Trump's wife seemed to echo some of his claim that there is a conspiracy out to derail his campaign.
"I was not surprised that the tape came out," she said.
"Why after so many years? Why three weeks before the election?"
"It was the media. It was NBC. It was Access Hollywood."
Ms Trump said the "left-wing media" was "bashing him because they want to influence the American people how to vote. And they're influencing in the wrong way."
She voiced dismay when told that most Americans believe the Republican presidential candidate has made unsolicited and unwanted passes at women.
"My husband is kind, and he's a gentleman. And he would never do that," she said.
"Everything was organised and put together to hurt him, to hurt his candidacy."
Bush leaves role at TV network
Mr Bush, who had joined NBC's morning flagship Today programme in August, is now leaving the network.
NBC News said last night he was "leaving the show effective Monday, according to an internal memo sent to staff by Senior Vice President Noah Oppenheim, who oversees the broadcast".
"I am deeply grateful for the conversations I've had with my daughters, and for all of the support from family, friends and colleagues. I look forward to what lies ahead," he said in a statement.
The presenter was making $3.5m a year in his current position at NBC, hosting an hour of morning news heavy on entertainment content.
