A "hidden" legion of voters could come out of the political shadows for the first time to deliver Donald Trump the presidency, despite many refusing to admit they support the controversial nominee.

That's according to early voting indications and political theory based on the "Bradley effect", which claims many voters have made up their minds to support Trump but won't admit to doing so for fear of being branded politically incorrect. It's named for former Los Angeles Mayoral candidate Tom Bradley who failed to get elected despite polls putting him in the lead.

With polls showing Mr Trump leading for the first time since May, Chatham House's head of the America's program Xenia Wicket said there's no doubt Trump has inspired those who wouldn't have voted otherwise.

"We already know that is going to be the case. If you look at the people Trump has brought to rallies, they're not your typical Republican voter. Even if you assume a vast proportion might not actually vote, which is probably an error in judgment - these are visible people. We know who they are, we've seen them."

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"Clinton supporters are not quite so enthusiastic ... That's one of the problems because Obama was able to bring out huge numbers who hadn't voted in the past and based on early voting numbers it looks like they're not coming out. How does one reharness the voters he had? This really is all about enthusiasm at some level."

President Barack Obama speaks to supporters while campaigning for Democratic presidential candidate Hillary Clinton. Photo / AP
President Barack Obama speaks to supporters while campaigning for Democratic presidential candidate Hillary Clinton. Photo / AP
Democratic presidential candidate Hillary Clinton greets workers. Photo / AP
Democratic presidential candidate Hillary Clinton greets workers. Photo / AP

It's the same phenomenon seen in the UK Brexit referendum, where around three million people who had not previously voted came out in enough numbers to deliver a Leave victory. Former Prime Minister David Cameron's spin chief Sir Craig Oliver has said assuming they would stay home was a major mistake.

Trump supporters have consistently maintained polls are missing his support base. House Speaker Newt Gringrich said: "I also believe that just as with the Brexit vote in Britain, he is in fact very likely to end up in a position where there's three or four percentage points that won't say his name."

Republican Speaker Paul Ryan has confirmed he voted for Donald Trump despite refusing to back him publicly. Another Virginia Republican told Politico "there is definitely a Bradley effect".

"I personally know many Republicans that won't admit that they are voting for Trump. I don't like admitting it myself. It won't matter if Hillary is up more than five points, but we might be in for a surprise if Hillary's lead is less than five points on Election Day," he said.

Research from the Brooking Institute shows the white working-class voters who form a large part of Trump's support base have a "high ceiling of potential voting power".

The group currently has turnout rates of 55-60 per cent compared to their college educated counterparts at around 75-80 per cent, meaning even a small increase in turnout could drive a boost for Republican numbers.

The Republican primaries also showed a boost in turnout, which rose to 15 per cent from 11 per cent four years ago. In contrast, Democratic numbers slumped from 20 per cent in 2008 under Obama to 15 per cent in 2016, according to Fair Vote.

With 21 million votes already cast in this election there is also evidence of higher engagement on the right, according to University of Florida's Michael McDonald who runs the United States Election Project.

He said several states show participation well above 2012 records and "the real story is not whether or not Democrats' level of engagement changed, but rather the higher levels of Republican engagement".

"Despite surprises, there appears more stability in the early vote than volatility. I suspect that most people have made up their minds about the candidates and the media frenzy is another piece of information to be thrown on the heap of what we already know about the candidates," he said in his latest blog.

Ms Wickett agreed little but a "black swan" event would change the minds of many voters, but said there was a larger chunk who claimed to be undecided than in previous years.

"The reality is many of those may be actually decided, they just haven't admitted it to themselves or anyone else," she said.

"The percentage of Americans who come out to vote in a typical presidential election is in the 40s so at some level anything that motivates voters to come out and engage in a Democratic process is a good thing. End of story."