WASHINGTON — The first contests of the 2008 presidential campaign have led to a dramatic shake-up in public opinion nationally, with Sen. John McCain now leading the Republican field and Sen. Barack Obama all but erasing Sen. Hillary Rodham Clinton's once-overwhelming advantage among Democrats, according to a new Washington Post-ABC News poll.

As the campaigns head into the next round of voting this week, the competitive contests in both parties have captured the public's attention. Four in five are closely tuned in, and a third are "very closely" following the races, a sharp increase from a month ago, and well higher than the proportions saying so at this stage in 2000 and 2004.

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.

In the new poll, 42 percent of likely Democratic voters support Clinton, and 37 percent back Obama. Clinton's support is down 11 percentage points from a month ago, with Obama's up 14. Former Sen. John Edwards held third position with 11 percent, followed by Rep. Dennis Kucinich of Ohio at 2 percent.

The big gains by McCain, 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 Rudy 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 percent and 19 percent, respectively.

Former Sen. Fred Thompson of Tennessee registers 8 percent, in single digits for the first time, with only half the support he had in early November. Rep. Ron Paul of Texas, who got 10 percent of the votes in Iowa and 8 percent in New Hampshire, is at 3 percent; Rep. Duncan Hunter of California is at 2 percent.

The sudden turnaround in national sentiment partly reflects the continued uncertainty among Republican voters about their field of candidates. Although McCain sits atop the GOP field, only a third of his supporters back him "strongly." And this week's primaries may further unsettle the race. Victories by McCain over Romney in Michigan tomorrow and in Saturday's South Carolina GOP primary, where his main rival appears to be Huckabee, would stamp McCain as the front-runner, but stumbles in either contest could further disrupt the GOP nomination battle.

In the Democratic race, opinions shifted decisively after Obama's big win in Iowa and Clinton's narrow victory in New Hampshire.

Despite the dip in support for Clinton, 68 percent of those backing her are "strongly" behind her. But Obama's support has both grown and deepened: Fifty-six percent are solidly behind him, up from 41 percent last month.

Clinton continues to lead Obama among Democrats, though by a slimmed-down, 8-point edge, while Obama has a 13-point edge among independents. Independent voters helped the senator from Illinois win Iowa and broke heavily for him in New Hampshire. Many of the upcoming contests limit participation to registered Democrats, which Clinton's advisers see as an advantage.

Obama holds a lead of nearly 2 to 1 among African-Americans, whose influence will be fully felt in the Jan. 26 Democratic primary in South Carolina, where nearly half of 2004 primary voters were black. In the new poll, 59 percent of African-American women support Obama and 35 percent back Clinton. Among white women, Clinton's margin over Obama is 20 percentage points.

Obama now leads Clinton among men, 42 percent to 33 percent, and while she retains an advantage among women, it has been cut. She has an 11 percentage-point lead among women, down from 39 points a month ago. In both Iowa and New Hampshire, women made up 57 percent of Democratic voters, according to election day polls.

In the Republican campaign, McCain's success in New Hampshire has translated into across-the-board gains among Republicans and GOP-leaning independents. By a 2-to-1 margin over Giuliani, McCain is seen as the candidate with the best experience to be president. He also tops the field on electability and leadership for the first time.

Conservatives remain split in their choice of candidate, with 25 percent supporting McCain, 23 percent Huckabee, 17 percent Romney and 16 percent Giuliani. McCain, never the darling of the right, has a wide lead among moderates and liberals. Three in 10 white evangelical Protestants support Huckabee, but 25 percent now back McCain, up from 12 percent over the past month.

The poll was conducted Jan. 9 to 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.