In what is bound to raise concerns in US Democratic Presidential candidate Barack Obama's [Images] camp, days ahead of the party's Denver Convention, Republican nominee John McCain [Images] has closed the gap with his rival, with the latest poll showing the two in statistical dead heat.

The latest Wall Street/NBC poll finds that 45 per cent of voters surveyed favour Obama while 42 per cent have inclination towards McCain -- the three point lead is down from six points a month ago.

The margin of error is given to be 3.1 per cent, meaning that Obama's lead could range from zero to six percentage points.

The poll points to a big challenge for Obama as his party gathers in Denver next week for its convention.

Another worrying statistic for Obama that has emerged from the poll is that only half of those who voted for Hillary Clinton [Images] in the primaries say they are now supporting Obama and one in five is supporting McCain.

With 11 per cent of all voters saying that they would vote for Senator Clinton if she were running against McCain, Obama's campaign faces the critical task of bringing his vanquished Democratic rival's supporters to his side.

The Republican has reached out to Clinton supporters by offering steady praise for the former first lady and hinting that he would be open to a running mate who supports abortion rights.

The poll has some cautionary notes for McCain as well. It shows that his supporters are much less enthusiastic than Obama's, and finds widespread concern among voters about his age.

But his performance, the Journal said, is impressive given the built-in advantages Democrats enjoy this year, including President George W Bush's [Images] unpopularity and the weak economy.

Three in 10 in the new poll say only McCain is running a 'negative campaign', versus just 5 per cent who say the same about Obama. Still, this doesn't seem to be hurting McCain, reinforcing the notion that in politics, attacks work, the paper said.

By 52 per cent to 27 per cent, voters say McCain would be better than Obama on 'international crises such as Iran or Russia [Images] and the nation of Georgia'.

But nearly four in 10 voters said they are concerned that McCain, who turns 72 next week, is too old to assume the office.

Given a chance to choose among third-party candidates Ralph Nader and Bob Barr, in addition to the two major-party candidates, a small portion did just that, most of them coming from McCain's column.

In a four-way race, Senator Obama kept 95 per cent of his supporters while McCain could retain just 90 per cent of his, a sign that some of the McCain voters are simply supporting him because they don't like the alternative.