The Pennsylvania primary defeat and renewed furor over his former pastor's anti-American rhetoric has led to a fall in the ratings of Democratic presidential hopeful Barack Obama, a new survey has found.

Obama, who still leads Hillary Clinton in the race for the Democratic presidential nomination, however, finds himself facing plummeting numbers.

According to New York Times/CBS survey released on Wednesday, 51 per cent of Democratic primary voters now expect Obama to win the nomination, down from 69 a month ago.

At least 48 per cent of Democrats now say Obama is the candidate with the best chance of beating Senator John McCain, the presumptive Republican nominee, down from 56 per cent a month ago.

The poll, which was conducted on April 25-29, before Obama's news conference on Tuesday in which he denounced his former pastor, the Reverend Jeremiah A Wright Junior, may not have fully captured the impact of the controversy or Obama's response.

The poll shows the Illinois Senator still holds an edge over his rival Hillary Clinton on several key measures. As many as 46 per cent of the Democratic primary voters said he remained their choice for the nomination, while 38 per cent preferred Clinton, down from 43 per cent last month.

Clinton has further lost support among men in recent weeks, it added.

Obama also has an advantage over Clinton in ratings on honesty and integrity and in being less beholden to special interest groups.

But, the Times said a month of upheaval, including a nearly 10-point loss to Clinton in Pennsylvania, has taken a toll with 56 per cent of Democrats describing their party as divided.

In contrast, 60 per cent of Republicans see their party as unified, a striking turnaround from the Republican turmoil at the start of the primary season.

The deteriorating economy continues to trouble the voters, with more than 4 in 10 citing the economy as the one issue that they want the candidates to address, up from about 30 per cent in a CBS News Poll in mid-March.

Democrats see no early end to the Obama-Clinton battle, the poll found.

Approximately 7 in 10 Democratic voters predict that their party's nominee will not be decided before the convention in August. And most voters say that this will eventually hurt their party's chances against McCain.

Still, there is resistance to the idea of party leaders stepping in to resolve the fight. Even among Democrats who said a lengthy battle would hurt the party, a majority said the contest should continue, until one candidate clearly wins the delegate count.

The survey suggests a very competitive race this November, regardless of whom the Democrats nominate. In a head-to-head race between Obama and McCain, both candidates are backed by 45 per cent of the registered voters.

In a race between Clinton and McCain, 48 per cent back Clinton while 43 per cent support McCain.

President Bush continues to get low marks on his overall job performance, with just 21 per cent approving of his handling of the economy.

A majority of voters said they preferred that the next president try to end the war in Iraq within the next few years.