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. Read More