In a new Washington Post-ABC News poll , a majority of Republicans -- 52% -- say that they believe then-President Barack Obama spied on then-candidate Donald Trump during the 2016 presidential campaign. 52%!

The claim originates with, natch, Trump, who tweeted in early March that he had just been informed that Obama had wiretapped Trump Tower during the 2016 campaign.

Terrible! Just found out that Obama had my "wires tapped" in Trump Tower just before the victory. Nothing found. This is McCarthyism!

Despite promises to do so, Trump has provided zero evidence to back up that claim. And it has been denied on the record by, among other people, FBI Director James Comey, former Director of National Intelligence James Clapper and the heads of both the House and Senate intelligence committees. Numerous fact-checkers have also found absolutely no factual basis for what Trump tweeted that early March morning.

No matter! Trump said it, it makes Obama look bad/plotting/evil and, for a decent chunk of people, that's enough. (Overall, just more than three in 10 adults said that Obama had spied on Trump during the election).

This quote, taken from the Post story on the poll results , is telling. "I wouldn't put it past the Obama bunch," said Gary Phillips, a Trump supporter in Ohio. "I wouldn't believe (Obama) if he said today was sunny or today was cloudy."

Under this logic, ANY accusation leveled at Obama by Trump -- like, say, that he wasn't born in the United States -- will find fertile soil within a not-insignificant segment of the Republican base. (This is the same logic, by the way, by which Trump has successfully disqualified the media in the eyes of many Republicans; they are bad people, so what they report is, by necessity, dishonest and wrong.)

Democrats aren't immune from this sort of thinking either, the poll shows. More than six in 10 Democrats believe that Russia tried to sway the election with the help of the Trump campaign.

To be clear: There is no evidence of that sort of collusion. But, while there is no proof of fire, there is lots and lots of smoke surrounding the Trump campaign's ties to Russia -- including previously undisclosed meetings with the Russian ambassador by Attorney General Jeff Sessions and former national security adviser Michael Flynn. Plus lots of other connections -- Carter Page, Roger Stone, etc. -- that are even murkier. And an active FBI investigation into Russian meddling.

The Trump allegations about wiretapping have none of that. Just the President of the United States tweeting from Mar-a-Lago. And yet, a majority of Republicans believe him -- because it plays into the conclusions they've already drawn about the other side. Of course Obama spied! He can't be trusted!

Those sorts of evidence-free opinions are dangerous to a healthy democracy. Which makes their preponderance these days all the more troubling.