Democratic presidential candidate Hillary Clinton has sunken to third place among independent voters, a new poll reveals.

Clinton pulled just 22 per cent of the independent vote in the latest Fox News poll, finishing behind Republican presidential candidate Donald Trump and Libertarian Party candidate Gary Johnson.

Trump led among independents, pulling 32 per cent of the vote, while Johnson, a former governor of New Mexico, got 23 per cent.

Democratic presidential candidate Hillary Clinton pulled just 22 per cent of the independent vote in the latest Fox News poll

'This is very bad news for Hillary Clinton,' Republican strategist Susan Del Percio told the New York Post. 'She needs independents to win in November.'

Clinton did, however, show a three-point national lead overall in the poll, which was released on Friday.

The poll showed Clinton leading 42 per cent to Trump's 39 per cent in a theoretical match up.

Del Percio noted that the poll showed a drop in overall support for Trump.

'Both candidates must start to broaden their support beyond their base, and hope that Governor Johnson's campaign on the Libertarian line does not gain steam,' she told the Post.

Tim Malloy of the Quinnipiac University Poll told the Post that those voting for Johnson could be pivotal in the upcoming election.

'Don't underestimate the importance of the Libertarian vote,' he said. 'They are not on the fence - they are not going to budge. And they could make all the difference in a close election.'

Clinton's presidential campaign postponed its first joint event with President Barack Obama on Wednesday in Green Bay, Wisconsin, because of the deadly shooting in Orlando, Florida.

Clinton's campaign said on Sunday that the event will be rescheduled.

She finished behind Republican presidential candidate Donald Trump and Libertarian Party candidate Gary Johnson (left). Trump led among independents, pulling 32 per cent of the vote, while Johnson (pictured), a former governor of New Mexico, got 23 per cent

Clinton and Obama were scheduled to make their first appearance together since the president announced his endorsement of the likely Democratic nominee last week.

She opened her general election campaign this weekend by casting herself as a uniting force against the divisiveness of GOP rival Trump.

The first general election ad released by her campaign splices clips of Trump threatening protesters and mocking a disabled reporter with scenes of Clinton visiting factories, greeting diverse groups of voters and stepping off a plane as secretary of state.

She ended the one-minute spot saying: "What kind of America do we want to be? Dangerously divided or strong and united? I believe we are always stronger together."

Trump responded Sunday on Twitter: "Clinton made a false ad about me where I was imitating a reporter GROVELING after he changed his story. I would NEVER mock disabled. Shame!"