A mountain of evidence points to a single fact: Russia meddled in the U.S. presidential election of 2016.

In both classified and public reports, U.S. intelligence agencies have said Russian President Vladimir Putin ordered actions to interfere with the election. Those actions included the cyber-theft of private data, the placement of propaganda against particular candidates, and an overall effort to undermine public faith in the U.S. democratic process.

Members of Congress, both Democrats and Republicans, have held open and closed door hearings to probe Russia's actions. The congressional investigations are ongoing.

Facebook, Google and Twitter have investigated their own networks, and their executives have concluded — in some cases after initial foot-dragging — that Russia used the online platforms in attempts to influence the election.

After all this, one man keeps saying it didn't even happen.

"This Russia thing with Trump and Russia is a made-up story. It's an excuse by the Democrats for having lost an election that they should've won," said President Donald Trump in an interview with NBC's Lester Holt in May.

On Twitter in September, Trump said, "The Russia hoax continues, now it's ads on Facebook. What about the totally biased and dishonest Media coverage in favor of Crooked Hillary?"

And during an overseas trip to Asia in November, Trump spoke of meeting with Putin: "Every time he sees me, he says, 'I didn't do that.' And I really believe that when he tells me that, he means it." In the same interview, Trump referred to the officials who led the intelligence agencies during the election as "political hacks."

POLITIFACT: Notable misstatements about Donald Trump from 2017

Trump continually asserts that Russia's meddling in the 2016 election is fake news, a hoax or a made-up story, even though there is widespread, bipartisan evidence to the contrary.

When the nation's commander-in-chief refuses to acknowledge a threat to U.S. democracy, it makes it all the more difficult to address the problem. For this reason, we name Trump's claim that the Russia interference is a hoax as our Lie of the Year for 2017.

Readers of PolitiFact also chose the claim as the year's most significant falsehood by an overwhelming margin.

It seems unlikely — though not impossible — that Russia interference changed the outcome of the election. We at PolitiFact have seen no compelling evidence that it did so.

Trump could acknowledge the interference happened while still standing by the legitimacy of his election and his presidency — but he declines to do so. Sometimes he'll state firmly there was "no collusion" between his campaign and Russia, an implicit admission that Russia did act in some capacity. Then he reverts back to denying the interference even happened.

It's not so much that Trump trades in falsehoods — it's more that he tries to create a different version of reality simply by asserting it.

That denial is of a different order from most presidential posturing, said Nicholas Burns, who served as ambassador to NATO under President George W. Bush.

"I've worked for both parties," Burns said during public testimony to the Republican-controlled Congress this summer. "It's inconceivable to me that any of President Trump's predecessors would deny the gravity of such an open attack on our democratic system.

"I don't believe any previous American president would argue that your own hearings in the Senate are a waste of time or, in the words of President Trump, a witch hunt. They're not; you're doing your duty, that the people elected you to do."

Seriousness of Russian interference

Countries have meddled in each other's internal politics before. But 2016 was different.

New cyber tools had come online, with Facebook putting robust sharing options in the hands of its users. In previous presidential cycles, online organizing had been innovative or unusual. In 2016, the sharing of political information online was widespread, cheap and easy to execute.

Meanwhile, Russia was concerned about its standing after conflicts with the former Soviet republic of Georgia. International sanctions resulted from Russia's 2014 annexation of Crimea, which Putin wanted to see lifted.

POLITIFACT: Looking back at the Trump team's falsehoods of 2017

Putin also had particular animosity toward Hillary Clinton, the Democratic nominee who had served as secretary of state. Putin openly blamed Clinton for inciting mass protests against his regime in late 2011 and early 2012. A publicly available intelligence assessment said that Putin also "holds a grudge for comments he almost certainly saw as disparaging him."

In July 2016, Wikileaks released thousands of emails stolen from the Democratic National Committee. The release led to DNC chair Debbie Wasserman Schultz stepping down after grassroots activists accused her of favoring Clinton over Bernie Sanders in the Democratic primary. Both intelligence officials and cybersecurity specialists concluded the hack had all the marks of a Russian operation. In October, Wikileaks began publishing the emails of Clinton campaign chair John Podesta.

Meanwhile, media websites connected to Russia, such as RT and Sputnik, spread suspicious or even fake news reports during the election, aided by online trolls and bots. Sputnik published an article that claimed Podesta's email included incriminating information about events in Benghazi, an allegation that turned out to be incorrect. (Trump himself repeated this false story.)

Russian hackers attempted to get into the computer systems of local elections officials around the country. The attempts never penetrated vote tallying systems, according to federal agencies, but the federal government warned local officials to redouble efforts to secure their systems.

Obama failures

If Trump wanted to address the Russia controversy head on, he could argue that he was correcting a lack of action on the part of the Obama administration, which learned in summer 2016 that election-related hacks could be traced back to the Russian government.

President Barack Obama personally confronted Putin at the September G20 Summit in China; other high-level contacts took place as well. Back home, Homeland Security Secretary Jeh Johnson warned state-level elections officials to protect voting-related systems from cyber intrusions.

But the overall response of the Obama team before the election was muted — perhaps due to concerns about the appearance of playing politics, or because they thought Clinton would be the certain winner of the election.

As it turned out, the most significant actions the Obama administration took happened after the election. In late December, Obama ordered 35 Russian diplomats and suspected operatives to leave the United States, and he ordered more sanctions.

Those actions have reverberated into the Trump administration.

Soon after Obama moved against Russia, Michael Flynn, then named as Trump's incoming national security adviser, contacted Russian ambassador Sergey Kislyak. Flynn asked that Russia not escalate the situation with a specific response to Obama's actions.

Later, in January 2017, Flynn told the FBI falsely that he hadn't spoken about sanctions with Kislyak. That wasn't true. On Dec. 1, Flynn made a plea deal with special prosecutor Robert Mueller, pleading guilty to one count of making false statements to federal investigators.

Meanwhile, technology companies spent 2017 investigating how much activity from Russian interests unfolded on Facebook and Twitter. Facebook estimated that 126 million people were served Russia-influenced content during the two-year period before the election.

"The Russians tried to use our tools to sow mistrust," said Facebook CEO Mark Zuckerberg in November. "We build these tools to help people connect and bring us closer together. They used them to try to undermine our values. What they did is wrong, and we're not going to stand for it."

Congressional committees have in recent months released some of the ads that Russia-affiliated groups placed on Facebook; they seemed aimed at stoking anger on both the right ("I want Sharia law banned in every state across America!") and the left ("Black Panthers were dismantled by US government because they were black men and women standing up for justice and equality").

Some ads were overtly anti-Clinton. One depicted an image of Jesus Christ boxing Hillary Clinton wearing devil's horns under the words " 'Like' if you want Jesus to win!"

As 2017 has marched on, a key point that Trump and his team once insisted upon has proved false. After first claiming that no one from the campaign met with the Russians, they've admitted to several meetings, including a June 2016 meeting attended by Trump's son, Donald Trump Jr.; then-campaign chairman Paul Manafort; and Trump's son-in-law, Jared Kushner.

Trump's obsession with fake news

Trump's labeling of the Russia story as a hoax fits in with his pattern of dismissing critical coverage as "fake news." He's used the term when he believes his administration doesn't get complimentary coverage, such as hurricane cleanup in Puerto Rico, or even when his comments have been reported accurately, such as his remarks about white supremacists in Charlottesville, Va.

Since the beginning of 2017, President Trump has publicly invoked the phrase "fake news" more than 170 times. Virtually every instance has been in response to critical news coverage.

Trump has insisted there was no collusion between his campaign and Russia. Some legal experts have said that it would be far-fetched to think the Trump campaign has broken laws.

"Even if it were to turn out that the Trump campaign collaborated, colluded or cooperated with Russian agents, that alone would not be a crime, unless the campaign asked them or helped them to commit criminal acts such as hacking," said attorney Alan M. Dershowitz in an op-ed for the New York Times.

But Trump has gone further than simply saying the campaign didn't break laws. He has said the whole story is fake.

It's that characterization — that Russia's interference in the election doesn't even exist — that is contradicted by a mountain of evidence, say foreign policy experts.

"There certainly has been some speculation in the media that has gotten ahead of the facts. That always happens. The media is littered with pundits who get paid to make speculation ahead of the facts," said Daveed Gartenstein-Ross, a senior fellow at the Foundation for Defense of Democracies, a right-leaning foreign policy think tank. "That doesn't make it fake news. That doesn't make the story as a whole 'fake news.' "

Investigations should go forward until all the facts come out about Russia's role in the election, said Benjamin Wittes, a senior fellow at the Brookings Institution and editor in chief of Lawfare, a national security blog.

"It's possible that the president obstructed justice. It's also possible the president behaved wildly inappropriately without obstructing justice," Wittes said. "And it's important to find out exactly what happened."

"Once you say, we don't mind foreign governments interfering with your elections, then you're on your way to yielding up significant aspects of sovereignty," he added. "It's an important line to defend."

Contact Angie Drobnic Holan at aholan@tampabay.com. Follow @AngieHolan.