"But there really is two weeks to go and in this era of instant news that can spread, anything can happen."

Just under two weeks from the election, voting has already begun in 27 states and will commence soon in another 12 states, both in person at public venues and via postal votes.

Democrats are reportedly outpacing early mail-in ballots in Florida and North Carolina compared to when Barack Obama defended his presidency in 2012.

Republican voters have requested slightly more mail-in ballots than Democrats, but not as many as when Mitt Romney lost Florida and narrowly won North Carolina four years ago.

Every major electoral college forecast has Mrs Clinton on track to clinch the 270 electoral college votes, with the forecast ranging from 278 to 352. AP

"Democrats have narrowed already the advantage that the Republicans had in 2012," said US Election Project analyst Michael McDonald, who tracks early and absentee voting patterns, said to Politico.

'Not encouraging' for Trump

Republican voters tend to be older and often live in distant rural areas, so are more inclined to mail in their ballots early.

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Democrats tend to easily outperform in early in-person voting, which ramps up next week.

David Kochel, a former strategist for Mr Romney, said Mr Trump had to win Florida and North Carolina.

"The early vote totals in North Carolina are not encouraging for the Trump campaign," he said.

On Tuesday, Republican pollster Frank Luntz tweeted that 658,177 Democrats had voted early (525,076 by mail and 133,041 in-person) in Florida, just behind 665,092 Republicans (556,058 mail and 109,034 in-person), in a bad sign for Republicans given that they historically have a larger lead early on.

Democrats are also buoyed by early voting trends in Nevada and Arizona, a traditional Republican state that is surprisingly in play for Mrs Clinton since Mr Trump's poll numbers plunged this month.

The Trump campaign has claimed it is seeing strong enthusiasm in Florida, whose crucial 29 electoral college votes will shape which candidate can reach the required 270 votes across the states.

It also is talking up its prospects of flipping Ohio and Iowa, which President Obama won in 2012.

New voters lean left

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The mail in ballots are not being opened until election day, so it's impossible to know for sure which party is winning. Generally, all that is known is the person's party of registration. About 90 per cent of Democrats intend to vote for Mrs Clinton and about 80 per cent of Republicans say they plan to back Mr Trump.

About 200 million Americans are registered to vote on November 8. About 60 per cent of eligible voters have participated at the past three elections.

Democrats have been encouraging new voters to register, such as young people and immigrants who tend to vote for the left of centre party.

Analysis by TargetSmart of 15 battleground states found that 42.6 percent of the new voters registered this year lean Democratic and 29 percent lean Republican, with the rest identifying as independents.

Every major electoral college forecast has Mrs Clinton on track to clinch the 270 electoral college votes, with the forecast ranging from 278 to 352.