WASHINGTON -  The race between Democratic Hillary Clinton and Republican Donald Trump has narrowed to essentially a dead heat nationally in a new poll, raising the stakes dramatically for the first Presidential debate on Monday night.

Likely voters split 46 per cent for Clinton and 44 per cent for Trump, with Libertarian Party nominee Gary Johnson at 5 per cent and Green Party nominee Jill Stein at 1 per cent, according to the latest ABC News/Washington Post poll. Among registered voters, Clinton and Trump are tied at 41 per cent, with Johnson at 7 per cent and Stein at 2 per cent.

In a two-way matchup between the major-party nominees, Clinton tops Trump by 49 per cent to 47 per cent among likely voters, and the two are tied at 46 per cent among all registered voters. Clinton’s two-point edge among likely voters, in both the four-way and two-way ballot tests, is within the survey’s 4.5 percentage-point margin of sampling error.

Meanwhile, an American professor, who has correctly predicted all the last eight Presidents of the United States, says Trump will win the November election.

Allan Lichtman, a distinguished professor of history at American University, told The Washington Post that based on a system he calls "Keys to the White House", Trump will emerge victorious.

“Based on the 13 keys, it would predict a Donald Trump victory,” he claimed.

The professor said he derived the system of true/false statements, "by looking at every American Presidential election from 1860 to 1980, and have since used the system to correctly predict the outcomes of all eight American Presidential elections from 1984 to 2012.”

He said one of the keys he uses to predict the winner is that the candidate is charismatic or not. Lichtman said Trump’s Democratic rival “doesn't fit the bill.”

Another factor he mentioned against Clinton is that she is going to run after President Obama — a Democrat - who had no major domestic or foreign policy achievements. He also explained that his prediction is not based upon the polls, adding, “the polls have very recently tightened.”

“Clinton is less ahead than she was before, but it's not because Trump is rising, it's because Clinton is falling,” he added.

According to the professor, the upcoming election was the most difficult to assess, because “We have never before seen a candidate like Trump, and Trump may well break patterns of history that have held since 1860.”

A vast 74 per cent of Americans plan to watch the debate, according to ABC News/Washington Post poll. And while eight in 10 say it won’t change their minds, that leaves more than enough to shift the balance in an increasingly closely fought contest, with unprecedented levels of qualms about both candidates.

Trump, in particular, is running competitively despite persistent doubts. Around six in 10 Americans continue to see him as unqualified, untrustworthy, temperamentally unsuited and insufficiently knowledgeable of world affairs to serve effectively as President. Yet he’s capitalising on strength in his core support groups and on Clinton’s own weaknesses, including concerns about her health.

In all, 44 per cent of likely voters state they’d vote for Trump if the election were today, numerically his best since spring. Forty-six per cent prefer Clinton, unchanged from an ABC/Post poll early this month and virtually unchanged since June. The 2-point gap between them is not significant, given the survey’s margin of sampling error.

The race has closed from an 8-point Clinton lead in early August.

Chiefly, though, Trump’s fortunes rest on his core supporters, white men who lack a four-year college degree, an economically stressed and socially and politically conservative group. He leads Clinton among them by 76-17 per cent, an enormous 59-point advantage. That’s widened from 40 points early this month; it’s a group Mitt Romney won by 31 points - half Trump’s current margin - in 2012.

Clinton, for her part, has advanced to a 25-point lead among college-educated white women, 57-32 per cent, extending her advantage in this group from 10 points early this month. Romney won them by 6 points in 2012.

The division between non-college white men and college-educated white women is its widest by far in exit poll data back to 1980. The outcome among these two groups, in turnout and preference alike, may well be a decisive factor in the election results.

Trump also leads among college-educated white men and non-college white women, by 11 and 12 points, respectively. As a result, he leads by 16 points among whites overall, while Clinton leads by 50 points among non-whites, a bit less than the typical Democratic margin. (One reason is that 6 per cent of non-whites favour Johnson or Jill Stein of the Green Party.) Further, Clinton now leads by 19 points among women, Trump by 19 among men - a yawning 38-point gender gap, triple the average gender gap in exit polls dating to 1976.

Other contours of the race also are without precedent, including both candidates personal unpopularity.About 59 per cent of Americans see Trump unfavourably; 55 per cent state the same about Clinton.

That said, Clinton’s unfavourable rating is a point from its record high, set in late August, while Trump’s is well down from its peak this year, 70 per cent in mid-June. Further, while neither candidate is seen as particularly honest and trustworthy, Trump has gained an advantage on this gauge among likely voters. Fewer than half, 45 per cent, see him as honest and trustworthy; but it goes lower, to 36 per cent for Clinton.

Clinton pushes back on other key factors. About 55 per cent of likely voters state she has the personality and temperament to serve effectively, 57 per cent say she’s qualified for the office and 68 per cent say she knows enough about world affairs. These drop to 41, 47 and 42 per cent, respectively, for Trump - albeit his best scores on these qualities to date.

Three-quarters, meanwhile, think Trump is in good enough health to serve as President.

That slides to 53 per cent for Clinton, likely given the recent bout with pneumonia that caused her to faint while leaving a 9/11 memorial event in New York.

Much of the race reflects two fundamentals: Partisanship and turnout. Among likely voters who are Republicans or GOP-leaning independents, 87 per cent back Trump; among Democrats and Democratic leaners, 84 per cent favour Clinton.

The strong pull of partisanship is one factor that boosts Trump despite compunctions about his suitability.