(CNN) President Donald Trump's former fixer and attorney Michael Cohen pleaded guilty in federal court Thursday to lying to Congress about his business talks with Russia during the 2016 presidential campaign.

The White House and Trump shot back that Cohen can't be trusted and the President alleged that Cohen is making things up to get a lighter prison sentence.

Who to believe is becoming a major element of what we know about special counsel Robert Mueller's methodical Russia investigation, which Trump for months has attacked as a witch hunt out to get him.

A majority (56%) of Americans said the things Trump says publicly about the Russia investigation are mostly or completely false, according to an August CNN poll conducted by SSRS . Only 37% thought the things he says about the investigation are completely or mostly true.

The only groups where a plurality said Trump tells the truth regarding the Russia investigation were those who approve of the job he's doing as President (77% of whom said that what he says about the investigation is true, compared with 14% who said it is false), Republicans (75% true, 19% false), conservatives (63% true, 31% false) and white non-college graduates (49% true, 46% false). Every other major demographic group or political subset said they think most of what he says regarding Russia is false.

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