President Barack Obama has lacerated Donald Trump over his claims the November election is rigged against him, telling the Republican to "stop whining and go try to make his case to get votes".
With opinion polls showing him falling further behind against White House rival Hillary Clinton, Trump has intensified his allegations, although numerous studies have shown voter fraud in US elections is rare.
At a campaign rally on Tuesday in Grand Junction, Colorado, Trump continued his attack, saying: "The press has created a rigged system and poisoned the minds of the voters."
But he also used his speech to propose term limits for members of Congress, six years for members of the House of Representatives and 12 years for members of the Senate.
Obama was asked about Trump's voter fraud assertions on Tuesday at a joint news conference following meetings with Italian Prime Minister Matteo Renzi.
He responded with a blistering attack on Trump, noting US elections were run and monitored by local officials, who may well be appointed by Republican governors, and saying cases of significant voter fraud were not to be found in American elections.
Obama said there was "no serious" person who would suggest it was possible to rig US elections, adding, "I'd invite Mr Trump to stop whining and go try to make his case to get votes."
At the Grand Junction rally, Trump said Obama should stop campaigning for fellow Democrat "Crooked Hillary" and "get out and work on jobs and work on the border".
Trump has for weeks raised the possibility of illegal activities that could tarnish the November election result, citing scant or questionable evidence and even as Republican lawyers called his assertions unfounded.
In a report titled "The Truth About Voter Fraud", the Brennan Center for Justice at New York University School of Law cited voter fraud incident rates between 0.00004 percent and 0.0009 percent.
Trump's vice-presidential running mate, Mike Pence, took a softer tone than Trump during a campaign appearance in North Carolina, saying he was "confident" the integrity of the election would be ensured.
But Pence said "in recent years we've had instances, proven instances of voter fraud".
With three weeks left until election day and early voting under way in many states, Trump has been grappling with the fallout from the release of a 2005 videotape in which he brags about groping women.
He has said the remarks were just "locker room" talk and has strongly denied allegations by a string of women who have come forward since the video emerged to say Trump groped or touched them inappropriately.
The controversies will be the backdrop for the third and final presidential debate on Wednesday night (Thursday morning AEDT) in Las Vegas.
At Tuesday's news conference, Obama also criticised Trump for his "continued flattery" of Russian President Vladimir Putin, saying "it is out of step with" the country.
Trump has praised Putin as a strong leader and on Monday suggested if he won he might meet the Russian leader before he is even sworn in as president.
