Democrat Barack Obama's big national lead over Hillary Clinton has all but evaporated in the U.S. presidential race, and both Democrats trail Republican John McCain, according a Reuters/Zogby poll released on Wednesday.

The poll showed Obama had only a statistically insignificant lead of 47 percent to 44 percent over Clinton, down sharply from a 14 point edge he held over her in February when he was riding the tide of 10 straight victories.

Illinois Sen. Obama, who would be America's first black president, has been buffeted by attacks in recent weeks from New York Sen. Clinton over his fitness to serve as commander-in-chief and by a tempest over racially charged sermons given by his Chicago preacher.

The poll showed Arizona Sen. McCain, who has clinched the Republican presidential nomination, is benefiting from the lengthy campaign battle between Obama and Clinton, who are now battling to win Pennsylvania on April 22.

McCain leads 46 percent to 40 percent in a hypothetical match up against Obama in the November presidential election, according to the poll.

That is a sharp turnaround from the Reuters/Zogby poll from last month, which showed in a head-to-head match up that Obama would beat McCain 47 percent to 40 percent.

"The last couple of weeks have taken a toll on Obama and in a general election match-up, on both Democrats," said pollster John Zogby.

Matched up against Clinton, McCain leads 48 percent to 40 percent, narrower than his 50 to 38 percent advantage over her in February.

"It's not surprising to me that McCain's on top because there is disarray and confusion on the Democratic side," Zogby said

Obama gave a speech on Tuesday rebuking his pastor, the Rev. Jeremiah Wright, for sermons sometimes laced with inflammatory tirades but said he could not disown him and it was time for Americans to bind the country's racial wounds.

The poll showed Obama continues to have strong support from the African-American community but that he is experiencing some slippage among moderates and independents.

Among independents, McCain led for the first time in the poll, 46 percent to 36 percent over Obama.

He was behind McCain by 21 percent among white voters.

Zogby attributed this to a combination of the fallout from Clinton's victory in Ohio earlier this month and the controversy over Wright's sermons.

"And, just the closer he gets to the nomination, the tougher questions whites ask about an African-American candidate," Zogby said.

The March 13-14 poll surveyed 525 likely Democratic primary voters for the match up between Clinton and Obama. It has a margin of error of plus or minus 4.4 percentage points.

For the match up between McCain and his Democratic rivals, 1004 likely voters were surveyed. It has a margin of error of plus or minus 3.2 percentage points.