The White House's press secretary found himself behind the 8-ball on Tuesday defending President Donald Trump repeated post-election claim that 'millions' of people voted illegally in November.

As reporter after reporter challenged him for evidence, Sean Spicer blurted out that the Trump administration might launch an investigation into the possibility.

'Maybe we will,' he told one journalist during his daily press briefing.

Pressed on what that could mean, a flustered Spicer insisted that he 'did not' leave the door open for a wide-ranging government probe – but then allowed that 'anything is possible.'

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White House press secretary Sean Spicer told reporters on Tuesday that 'maybe we will' investigate President Donald Trump's claim that millions voted illegally in November, allowing that 'anything is possible'

Trump told congressional leaders Monday night that he would have won the popular vote if millions of illegal immigrants hadn't fraudulently voted for Hillary Clinton

The president told a group of Republican and Democratic lawmakers Monday night that he would have won the popular vote if it weren't for between 3 and 5 million illegal votes, according to people who were present.

'He was having a discussion with some folks and mentioned something in passing,' Spicer told reporters on Tuesday.

Democratic nominee Hillary Clinton collected nearly 3 million more votes than Trump but lost decisively in the Electoral College because the largest part of her popular-vote advantage was concentrated in a small number of states with giant Democratic voter bases.

As disenchanted Democrats argued against the merits of the Electoral College system in the wake of his victory, Trump tweeted that '[i] n addition to winning the Electoral College in a landslide, I won the popular vote if you deduct the millions of people who voted illegally.'

He also claimed on Twitter that there was 'serious voter fraud in Virginia, New Hampshire and California,' and challenged the media to report on it.

Spicer punted a question on Tuesday asking for specific evidence of voter fraud in November, rather than a statistical probability that it happened.

Spicer faced round after round of hostile questions from journalists who smelled blood in the water after Trump revived his claims of voter fraud on a massive scale

NPR reported in late November that the Trump transition team had provided it with a 45-page list of alleged instances of voter fraud. The public radio network did not publish the document, and a White House spokesperson did not respond on Tuesday to a request for a copy.

Brian Fallon, a Clinton campaign official who likely stood to become her White House press secretary, vented Tuesday afternoon on Twitter.

'Here we go again,' Fallon wrote in a pair of tweets. '@PressSec backs up Trump on patently false statement – this time that voter fraud prevented him from winning [the] popular vote.'

'This is why WH [White House] reporters should never boycott Spicer's briefings. Public needs to see this WH's bogus claims subjected to daily scrutiny.'

Spicer reinforced on Monday that Trump's notion of millions of illegally cast votes is 'a longstanding belief that he's maintained.'

But NPR correspondent Mara Liasson challenged him about why the president wouldn't want to get to the bottom of it.

'If 3 to 5 million people voted illegally, that is a scandal of astronomical proportions,' she declared in the White House press briefing room.

'Doesn't he want to restore Americans' faith in their ballot system? Wouldn't he want an investigation of this? This is a huge, huge scandal.'

'We'll see where we go from here,' Spicer replied. 'But right now the focus that the president has is on putting Americans back to work.'

Trump warned repeatedly in October, including at this Cleveland, Ohio rally, that he faced 'a rigged election' because of illegally registered voters and dead people on voter rolls

The president made allegations of 'voter fraud' in November, after he won the election, tweeting that he lost the popular vote because of 'millions of people who voted illegally'

Spicer, who was the Republican National Committee's chief communicator before moving to the West Wing, also declined to say if he personally agrees with Trump.

House Speaker Paul Ryan told reporters Tuesday at the U.S. Capitol that he has 'seen no evidence to that effect,' referring to Trump's claims. 'I've made that very, very clear.'

The president often warned in the final month of the presidential campaign that voter fraud could cost him the election.

During a late October rally in Cleveland, Ohio, he called it 'a rigged election.'

Citing statistics from the Pew Research Center, Trump said that 'there are 24 million voter registrations in the United States that are either invalid or significantly inaccurate. A lot, right? 24 million. A lot.'

'There are 1.8 million dead people that are registered right now to vote,' he continued.

'And folks, folks: Some of them vote! I wonder why. I wonder how that happens. They woke up from the dead, and they went and voted!'

'There are 2.8 million people who are registered in more than one state,' he mused. So, "We'll vote here, let's ride down the road, let's vote next to it".'

And then, reacting to a fan in the audience, he stopped cold.

Brian Fallon, who would likely have had Spicer's job if she had won the White House, tweeted Tuesday that Trump's view is 'patently false' and slammed Spicer for repeating 'bogus claims'

'Maybe they'll vote for Trump. I don't know. Maybe I shouldn't be saying this. I may be hurting myself!'

'You're right! You're right,' he responded to the unsolicited advice. 'Maybe they're gonna vote for Trump!'

'Alright,' he laughed, 'let's forget that. It's okay for them to do it!'

In a late November conference call with reporters, Trump's senior policy adviser Stephen Miller declined to speculate about a Justice Department investigation, but called voter fraud allegations 'an issue of concern.'

'There's a concern that so many voted who were not legally supposed to,' he said.

On Tuesday, Spicer defended his boss by citing the same Pew study then-candidate Trump crowed about three months ago, but confused its statistics with another piece of research.

'There was one that came out of Pew in 2008 that showed 14 per cent of people who've voted were non-citizens,' he claimed.

That number came from a 2014 Washington Post article by a pair of Old Dominion University professors. It was later challenged by other researchers, who claimed the professors' findings stemmed from 'measurement error.'