WASHINGTON — Inch by inch, voter by voter, Barack Obama and John McCain labored for more than a year to lock down supporters and woo defectors. It turns out, though, that the nation’s voters were a lot more fickle than commonly expected, and far more prone to switch allegiances.

An Associated Press-Yahoo News poll that tracked the same group of about 2,000 adults throughout the long campaign reveals a lively churning beneath the surface as people shifted their loyalties — some more than once.

Count Republican Kelly Townsend of Abilene, Texas, among them.

Townsend, 50, saw McCain as a “good person,” and supported him early on. But eventually she abandoned the Arizona senator in favor of Obama.

As for McCain, she concluded, “I just think he’s hot-tempered and a little bit of a warmonger.”

Over the long haul, 17 percent of those who eventually voted for Obama had expressed support for McCain at least once in a series of 10 AP-Yahoo News polls conducted since November 2007, before the party primaries began. And 11 percent of McCain’s eventual supporters had backed Obama at least once.

Conventional political wisdom says it can be pretty much taken for granted that most voters lean sharply left or right and commit to one candidate early on, and the real campaign fight is over a small slice of undecided voters in the middle.

But like much of the conventional wisdom in this anything-but-typical election year, that may be wrong. Election polls that showed only gradual shifts in support for Obama and McCain were masking a much more volatile electorate. Few voters made unwavering, long-term commitments to either candidate.

“In fact, things are a lot less locked down than they might appear in public,” said Bill McInturff, who was McCain’s campaign pollster. “And it’s why you don’t stop campaigning.”

Just 28 percent of those saying they voted for Democrat Obama, and 27 percent saying they backed Republican McCain on Election Day, said they would vote for that party’s candidate in all 10 AP-Yahoo News polls.

And just half of those surveyed supported their ultimate choice unwaveringly in AP-Yahoo News polls since June, when both parties’ nominees were known. Just six in 10 of both Obama’s and McCain’s voters backed their contender in those surveys without exception since September, when the general election campaign began in earnest.

That’s a contrast with most public opinion surveys. They showed Obama with a modest lead for much of the summer, McCain taking a slight edge after the two party conventions, then Obama building a telling advantage into the fall as the economic situation spiraled downward.

Those abandoning one candidate were often canceled out by others gravitating to him, resulting in little net change in the candidates’ overall support. Yet the frenetic, beneath-the-radar movement helps explain why the two political parties spent hundreds of millions of dollars this year. They needed to constantly woo new supporters while keeping those they thought they already had from defecting.

Stephen Ansolabehere, a Harvard University political scientist who has studied voting behavior, said such movement has been especially pronounced lately. He cited Republican defections because of unhappiness with President George Bush and the war in Iraq, uncertainty over which party could best address the economic meltdown and this year’s influx of young and other first-time voters.

“Right now there are a lot of voters who are kind of up for grabs on a long-term basis, not just for a short-term campaign,” he said.

Party labels, as always, had strong sway over how people voted, according to the AP-Yahoo News polls, conducted by Knowledge Networks. About three quarters of those calling themselves Democrats in November 2007 ended up voting for Obama, and about the same number in the GOP a year ago ultimately backed McCain.

“I felt he probably experienced the things I had, he had probably the loyalty to the country I feel,” Jim Miller, 60, of Silverton, Ore., a consistently loyal Republican, said of fellow Vietnam veteran McCain.

Yet only 46 percent of those identifying themselves as Democrats last November — and the same share of Republicans — stayed with their party in all 10 AP-Yahoo News surveys. Looked at another way, only four in 10 McCain voters called themselves Republicans every time while about the same number of Obama voters were unflinching Democrats.

“We were ready for change, obviously. He was the one who most looked like he’d do something for us,” said consistent Obama supporter Don Metzger, 43, of Sacramento, Calif.

Obama’s advantage in siphoning away votes from his rival was especially noteworthy in the campaign’s final months.

Nine percent of those saying in June that they supported McCain ended up voting for Obama. Only about half that many — 4 percent — of Obama’s June backers eventually voted for McCain.

It was similar in September. Just two months from Election Day, 7 percent of those saying they preferred McCain ended up voting for Obama. McCain won just 2 percent of the votes of those who said in September they backed Obama, according to the polls.

“At first I was like, Obama, Obama. I was sure I was going to vote for him,” said Carole Scoville, 27, of Pleasant Grove, Utah. She said she eventually voted for the Republican after she questioned the costs of Obama’s campaign promises and received e-mails about him, which included false rumors that Obama is Muslim.

Joel Benenson, Obama’s chief pollster, questioned the AP polls’ accuracy. He said his surveys showed less volatility than usual among voters because of the campaign’s high visibility and unusual length.

The AP survey also showed:

Only one in five who in November 2007 considered themselves independents stayed that way in all 10 AP-Yahoo news surveys. Those who were independents in that first survey ended up on Election Day dividing about evenly between the two candidates: 45 percent for McCain, 42 percent for Obama.

Three-quarters of those who said in January or April that they wanted Hillary Rodham Clinton to win the Democratic nomination voted for Obama.

Of those who voted on Election Day, 46 percent said in October they had been contacted and urged to vote by one or both campaigns. Those contacted voted 52 percent to 45 percent for Obama, and those not contacted voted 50 percent to 46 percent for Obama — virtually no difference.

The AP-Yahoo News poll was conducted 10 times from November 2007 through Election Day, and involved interviews with about 2,000 adults in each survey. The margin of sampling error each time was between plus or minus 2 percentage points and 3 percentage points.

The Election Day survey involved interviews with 1,989 adults and had a margin of sampling error of plus or minus 2.2 points.

The poll was conducted over the Internet by Knowledge Networks, which initially contacted people using traditional telephone polling methods and followed with online interviews. People chosen for the study who had no Internet access were given it for free.