Trump’s consistent tweeting—and the constant media coverage of those tweets—makes his favorite phrases familiar to the American public. And that familiarity could be key to making his claims seem plausible, even believable. By consistently referring to the investigation as a “hoax” and a “witch hunt,” Trump is framing the debate on his terms.

The actual ‘single greatest witch hunt of a politician’ in U.S. history

“If you start arguing about whether something is or isn’t a witch hunt, you’ve basically conceded that it’s plausible that it’s a witch hunt, or that that’s the right term to use,” says Tom Stafford, a cognitive scientist at the University of Sheffield in Britain. So when Trump says, over and over, that the Mueller investigation is a “rigged witch hunt,” people subconsciously begin to think the idea that it is a witch hunt could be plausible—even when there’s no evidence to substantiate that claim.

Public opinion polls, at least in part, bear out the theory that repetition leads to belief. A June poll from Politico and Morning Consult showed that 53 percent of Republicans have an unfavorable impression of Mueller, up 26 percent from July 2017. A July Ipsos poll showed that 75 percent of Republicans believe that the Mueller investigation is politically motivated against Trump, a number substantiated by a CBS News/YouGov Battleground Tracker poll that found that 70 percent of Republicans believe the investigation to be a “political witch hunt.” These opinion polls appear to show that a substantial portion of Americans not only believe the president’s claims about the investigation, but also that they’re comfortable adopting the specific language he uses to characterize it.

It’s not just the president’s tweets that are constantly repeating his “truths.” Media outlets like Fox News frequently broadcast his claims as trustworthy, something that is also almost certainly contributing to their plausibility among the segments of the population that trust the network. Eighty-five percent of poll respondents whose most trusted network was Fox News believed that the Mueller investigation was a witch hunt as early as June 2017, according to a Suffolk University poll—more than seven times the respondents for any other network. In their March 2018 poll, Suffolk found that just 35 percent of viewers who trusted Fox also trusted the Russia investigation to be fair and accurate.

These poll numbers all center around people whose political affiliations make them more likely to side with the president in the first place. Payne says that there are always at least two psychological forces at work: the force of familiarity, which is behind the illusory-truth effect, and the force of motivated reasoning, which is when people try to make new information fit in with their previously held beliefs. “In the case of President Trump making false statements about the Mueller investigation, the familiarity of repetition is working in the same direction for Republicans as their motivated reasoning is, so you have two things pointing in the same direction,” he said.