Story highlights Trump approval ratings hit new low at 39%

Majority polled believe Comey was fired to end FBI's Russia investigation

Washington (CNN) A majority of Americans believe President Donald Trump fired FBI Director James Comey last week in an effort to slow down or stop the investigation into the Trump campaign's ties with Russia, according to a new Monmouth University Poll out Thursday.

Nearly six in 10 Americans think it's likely that Trump dismissed Comey to hinder the investigation, the poll found. Just 20% believe the Trump administration's initial explanation that the firing was due to the way Comey handled the investigation into Hillary Clinton's use of a private email server. The poll finds 50% disapprove of Trump's firing of Comey while 36% approve.

The findings come on the heels of slipping approval ratings for the president. The president's job rating currently stands at 39% approval. Just two months ago, Trump's approval was 43%.

The poll also finds that the negative shift in Trump's job approval rating since March is concentrated in counties where the vote margin between Clinton and Trump in 2016 was less than 10 percentage points.

Trump's biggest decline came among people in swing counties -- 300 total where the winning margin between Clinton and Trump was in the single digits. In those counties, Trump's rating dropped from 41% approval and 46% disapproval in March to 34% approval and 54% disapproval now.

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