Clinton leads Trump 42-38, said the latest poll released by Public Policy Polling (PPP).
"Clinton certainly is favoured to win the Presidential race this fall," said Dean Debnam, president of the PPP.
"But it might not be the giant blowout it's been made out to be in some quarters. Trump has quickly gotten most rank and file Republican voters behind him and that has him positioned as a modest underdog for the general rather than a massive one," Debnam said.
In Florida and Pennsylvania, Clinton leads Trump by just one point, while the latter edges her in Ohio by four points.
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Interestingly, Senator Bernie Sanders - Democratic rival of Clinton - leads Trump by a few points in all these three States.
"Six months from Election Day, the presidential races between Clinton and Trump in the three most crucial states, Florida, Ohio and Pennsylvania, are too close to call," said Peter A Brown, assistant director of the Quinnipiac Poll.
"And the two candidates are about where their party predecessors were at this point in Ohio and Florida," he said.
"This election may be good for divorce lawyers. The gender gap is massive and currently benefits Trump," Brown added.
"In Pennsylvania, Clinton's 19-point lead among women matches Trump's 21-point margin among men. In Ohio, she is up 7 points among women but down 15 points with men. In Florida she is up 13 points among women but down 13 points among men.
"Trump would do a better job handling the economy, voters say. He also would do a better job handling terrorism, voters in Florida and Ohio say. Pennsylvania voters are divided," he said.