U.S. President Barack Obama lacerated Donald Trump on Tuesday over his repeated assertions that the Nov. 8 election is rigged against him, telling 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, Trump has intensified his allegations, although numerous studies have shown voter fraud in U.S. elections is rare.

Obama was asked about Trump's assertions on Tuesday at a joint news conference in the White House Rose Garden following meetings with Italian Prime Minister Matteo Renzi.

He responded with a blistering attack on the Republican candidate, noting that U.S. 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 American elections.

Obama said there was "no serious" person who would suggest it was possible to rig American elections, adding, "I'd invite Mr. Trump to stop whining and go try to make his case to get votes."

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.

The New York real estate developer currently trails Clinton by 7 points in national opinion polls, according to an average of polls compiled by RealClear Politics.

Regardless of what Obama said, the Trump campaign isn't backing away from its stance that the election is being "rigged" at his campaign stops Tuesday.

During a stop in Grand Junction, Colo., he said the media is "more crooked than crooked Hillary."

During the same speech, he said he doesn't believe the polls putting Clinton head of him in Colorado and is instead predicting he'll have one of the greatest victories in political history.

Trump and his running mate, Mike Pence say there are two reasons the election will be "rigged." One, the news media is colluding with Clinton by reporting that multiple women accuse him of sexually assaulting them. Trump additionally insists there will be fraud at the polls on election day.

Trump earlier advised his supporters to ignore mainstream news outlets and instead "read the internet."

String of controversies for Trump

Throughout the election campaign, Clinton has argued that Trump, with his divisive rhetoric and attacks against Muslims and other minorities, is not qualified to lead the United States.

Obama echoed that theme, arguing that 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, 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.

With three weeks left until Election Day and early voting both by mail and in person already under way in many states, Trump, who has never previously run for office, has had to fend off reports that he may not have paid any federal taxes over several years.

More dramatically, he has grappled with the fallout from the dissemination earlier this month of a 2005 videotape in which Trump was heard bragging about groping women and making other unwanted sexual advances.

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, denouncing his accusers as liars and fame-seekers.

3rd debate looms

The controversies will be the backdrop for the third and final of three presidential debates, to be held on Wednesday evening in Las Vegas.

Trump was widely viewed as having been on the defensive during the first debate and while he appeared to have improved his performance in the second debate, according to polling, Clinton was seen as the victor.

Trump's repeated warnings of a tainted election have raised concern over potential violence at polling sites and challenges to the result.

At Tuesday's press conference, Obama also criticized 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 that if he wins next month he might meet the Russian leader before he is even sworn in as president.

On Wednesday from 9 to 10:30 p.m. ET, Democrat Hillary Clinton and Republican Donald Trump face off in the third and final p​residential debate. The debate will be carried live on CBC News Network, CBC Radio One, live streamed on CBCNews.ca and on our CBC News Facebook page and YouTube channel.