By a substantial margin, Americans believe President Donald Trump pressured former FBI Director James Comey to end an investigation into whether Russia interfered in the presidential election.

The CNBC All-America Economic Survey of 800 adults across the country finds that 43 percent think the president pressured Comey, 26 percent believe he did not and 31 percent don't know enough to have an opinion or are unsure. The survey was conducted June 9-12, beginning the day after Comey testified before Congress, and has a margin of error of plus or minus 3.5 percentage points.

The survey also found the president has declining overall approval, but a better rating specifically on his handling of the economy.

The Comey result comes as NBC News and other news agencies have confirmed this week that special prosecutor Robert Mueller is inquiring whether the president obstructed justice by seeking to impede the Russian investigation. The president has called Comey a liar.

Democrats believe the president pressured Comey by 74 to 4 percent while Republicans believe he did not by a 52 to 14 percent. However, 34 percent of Republicans say they don't know enough to have an opinion or are unsure. The result reflects intensity over the issue among Democrats and to a lesser extent independents versus only lukewarm conviction from Republicans supporting Trump.

The investigation and other missteps by the president could be having an impact on his overall presidential approval rating. The CNBC All-America Economic Survey shows just 37 percent of the public approve of the job Trump is doing, with 51 percent disapproving. It's a 2-point decline from the April poll but comes amid what appears to be a troubling erosion of support among some key Trump support groups.

Republican approval dropped by 10 points from 82 percent in April. There was a similar 10 point decline among Southerners and whites without college degrees.

The Democratic pollster for the survey, Jay Campbell of Hart Research, said the declines are significant because a president typically needs an 80 percent approval rating or better from his own party to advance his agenda.

Tthe Republican pollster for CNBC, Micah Roberts of Public Opinion Strategies, cautioned against making too much of it. "The question becomes whether that drop is a blip or is starting a trend. We don't know that yet,'' he said.

Trump fares somewhat better among the public when asked about his handling of the economy, but there's been slippage there as well. Forty-one percent approve of his handling of the economy and 44 percent disapprove — a lower negative rating than his overall approval numbers. However, in April, Americans by a 44 percent to 41 percent margin approved of his job stewarding the economy.