The double-digit lead that Senator Barack Obama enjoyed over Senator Hillary Clinton three weeks ago, has evaporated with both candidates experiencing a decline in their image, according to a new national poll.

The latest Pew Research Center poll found that among registered Democrats and voters likely to lean Democratic, 47 per cent said that they favoured Obama, compared with 45 per cent who backed Clinton.

That's a marked difference from the results of the survey conducted in March, when Obama with 49 per cent support, had a 10 point lead over his rival for the Democratic presidential nomination.

The tightening Democratic race reflects a modest but consistent decline in Obama's personal image rather than improved impressions of Clinton, with fewer Democrats ascribing positive qualities to Obama.

Since late February, Obama's unfavourable rating has risen six points among all Democratic and Democratic-leaning voters. At the same time, Clinton's unfavourable rating among Democratic voters also has increased by seven points.

White working-class Democrats, in particular, express more skeptical views of Senator Obama, according to the survey based on phone interviews of 651 people, from April 23-27.

Despite the slippage, Obama's personal image remains highly positive and surpasses Clinton's on most dimensions. Large percentages of Democratic voters continue to see him as honest, inspiring, and down-to-earth.

Although Clinton made gains in overall support among Democrats, her credibility problem is, if anything, greater than it was a month ago. The proportion of Democratic voters describing Clinton as honest fell from 65 per cent to 57 per cent and the percentage describing her as phony increased from 29 per cent to 35 per cent.

Andrew Kohut, president of the Pew Research Center, told the NPR radio, "The slippage is powered by an even greater role of both race and class in the patterns of the answers."

For instance, Clinton's lead among white Democrats who did not attend college rose from 10 points in March to 40 points in the current survey.

"Democratic voters are very conflicted about these candidates. They're feeling worse about each of them. They're feeling worse about the nature of the race," said Kohut.

According to Pew Research, the unresolved Obama-Clinton race is wearing thin on an increasing number of Democrats, with 51 per cent of respondents saying that the fact that the race remains unsettled is a bad thing for the Democratic Party -- compared with 41 per cent in March and just 27 per cent in February.

Among black voters, Kohut said that Clinton's numbers resemble those of a Republican. "The black vote, since South Carolina, has gotten behind Barack Obama and stands behind him not only in the primaries, but now in these national polls," he said. "Voting along racial lines and along lines of education and income is very apparent in these primaries."

As for a general election, the latest poll suggests that in the eyes of the voters, neither Obama nor Clinton has a clear advantage over presumptive Republican nominee Senator John McCain. Both Democrats edged McCain by a small margin -- four or five percentage points.

Clinton runs better than Obama among core Democrats, while Obama does better among independents.

"So, each of them, overall, comes out about even, but comes out about the same in different ways," Kohut added.