Republican Donald Trump leads Democrat Hillary Clinton in the key US state of Ohio and only narrowly trails the former secretary of state in two other swing states, according to an opinion poll released Tuesday.

The survey by Quinnipiac University shows Trump ahead with the support of 43 per cent of voters to Clinton's 39 per cent in Ohio, a state considered a bellwether in US presidential elections in which the winner is determined based on states won rather than who receives the most votes nationwide. No Republican has ever won the White House without winning Ohio.

Clinton leads Trump in two other key states, Florida and Pennsylvania, but only by narrow margins. Clinton records 43 per cent to Trump's 42 per cent in both states.

However Clinton's rival in the ongoing Democratic primaries looks to be a stronger competitor against Trump. Bernie Sanders leads the Republican in all three states. He secures 47 per cent of the vote in Pennsylvania to Trump's 41 per cent, 44 per cent in Florida to 42 per cent for Trump and 43 per cent in Ohio to Trump's 41 percent.

The survey shows a tighter than anticipated race between Clinton and Trump, who both must overcome high unfavourable views among voters.

"Six months from Election Day, the presidential races between Hillary Clinton and Donald Trump in the three most crucial states, Florida, Ohio and Pennsylvania, are too close to call," Quinnipiac poll's assistant director Peter Brown said.

The pollster noted that since 1960 no candidate has won the presidency without winning at least two of the three states surveyed.

The poll surveyed around 1,000 voters in each of the states and had a margin of error of plus or minus three percentage points in each state.

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