WASHINGTON (AFP) - Hillary Clinton's lead in the United States presidential race against Donald Trump is evaporating with just over 50 days to go until election day, as she stirs ever less enthusiasm in her own camp.

The Democrat's average lead since late August is just 1.8 points at the national level, a drop of four points in two weeks, and in several key states where the November election is likely to be decided, Trump is ahead.

In Ohio the Manhattan mogul now leads the former secretary of state by 46 to 41 per cent, and in Florida by 47 to 44 per cent, although this falls within the margin of error, according to a CNN-ORC survey.

Clinton tried to project an air of calm as she jumped back on the campaign trail after a three-day enforced break due to pneumonia, saying she always said the race would be close.

But in a sign of jitters, her campaign announced that her former rival Bernie Sanders, who is popular among young people, and progressive senator Elizabeth Warren would campaign for Clinton this weekend in Ohio.

Meanwhile Trump declared: "We've had an incredible month. There is a great enthusiasm." It is not the first time the two candidates have been neck and neck. It happened briefly in May, before Trump lost ground.

But with the election less than two months away, Clinton enjoys ever less popularity among Democrats. Only 38 per cent say they are very enthusiastic about her candidacy, down from 47 per cent in August, according to a New York Times/CBS poll.

Trump's supporters appear much more fired-up: 55 per cent say they are very keen to vote, against just 36 per cent in the Clinton camp.

So for the Democrats, getting people out on Nov 8 will be key.

FiveThirtyEight, a website that analyses polls, historical and economic data, says Clinton still has a 60.1 per cent chance of winning, compared to 39.8 per cent for Trump.

Back on Aug 8, Clinton's chances stood much higher at 79.5 per cent, compared to 20.5 per cent for the Republican.

But since early last month Trump has overhauled his campaign team, tried to become more disciplined and less attack-oriented in his public appearances and has stopped insulting people. His new campaign chief, Kellyanne Conway, appears on television often to plug Trump.

"Trump had a couple of good weeks beginning with his success in Mexico" early this month, said Robert Shapiro, a political scientist at Columbia University in New York.

Shapiro added that Trump has been "rallying his base of support, saying more about policy issues, foreign policy, the economy, and... his proposal in terms of maternity leave and child care.

"The bar for evaluating him is not very high, but in that context he has been doing better in terms of campaigning and trying to look a little bit more presidential." For Clinton things have not been going so well.

She has been dogged for months by the controversy over her use of a private email server while serving as secretary of state and by attacks from her opponents against the Clinton Foundation.

Clinton has also taken heat for saying many Trump supporters belong in a "basket of deplorables." Shapiro said Clinton did not handle her bout of pneumonia well, either. The campaign's delay of two days in telling the public she was sick boosted a widespread perception that Clinton is not transparent.

But her recent bout with pneumonia itself does not appear to have scared away her supporters, according to a new Reuters/Ipsos opinion poll.

The Sept 9-15 tracking poll showed that 42 per cent of likely voters supported Clinton while 38 per cent backed Trump.
Democrats have reason to worry - but Clinton still has many ways to win the electoral college.

The presidential election is effectively the sum of 50 state elections, with each candidate gunning for a majority of 538 electoral votes divided among the states, all but two of which award all their votes to one candidate.

To reach 270 votes, Trump needs to carry a number of battleground states - Ohio and Florida but also Iowa, Virginia, New Hampshire, and North Carolina. But he must also lock in all the traditionally Republican states, several of which look as if they might snub his candidacy.

With Clinton dropping in the polls however, the looming first presidential debate on Sept 26 is not necessarily good news for her.

"Historically in these first debates, the incumbent president or the leading candidate tends to do less well on the first debate than the challenger," said Shapiro.

"And in this case, the expectations and the bar for Trump may be so low that he'll be evaluated differently than she will. The bar is higher for her than for Trump," he added.