Clinton had dominated in national polls from the outset, holding a 30- point advantage as recently as a month ago, but the competitiveness of the first two contests appears to have reverberated among Democrats across the country.

In the new poll, 42 percent of likely Democratic voters support Clinton (New York), and 37 percent back Obama (Illinois). Clinton's support is down 11 percentage points from a month ago, with Obama's up 14. Former senator John Edwards (North Carolina) held third place with 11 percent.

The big gains by McCain (Arizona), which come after his victory in the New Hampshire primary, mark the first time he has topped the Republican field in a Post-ABC News national survey.

His rise mirrors a dramatic tumble for former New York mayor Rudolph Giuliani, who led most national polls throughout 2007.

Giuliani, who finished well back in both Iowa and New Hampshire, ranks fourth in the new poll at 15 percent. McCain, meanwhile, has more than double the support he had a month ago and now stands at 28 percent among likely GOP voters.

Former Arkansas Gov. Mike Huckabee, who scored a big victory in the Iowa caucuses, and former Massachusetts Gov. Mitt Romney, the runner- up in both early contests, sit just above Giuliani, at 20 and 19 percent, respectively.

The poll was conducted Jan. 9-12 among a random national sample of 1,130 adults. The margin of sampling error for the full sample is plus or minus 3 percentage points.