MERRIMACK, N.H. — When politicians see polls they don't like, they recite a cliche: The only poll that counts is the one on Election Day.

Tuesday, the voters of New Hampshire proved the cliche right.

For days, poll after poll showed Illinois Sen. Barack Obama opening a big lead heading into the New Hampshire Democratic primary. But when the votes were counted, New York Sen. Hillary Rodham Clinton won. Even she seemed surprised.

Were the polls all wrong? Did the pollsters misjudge how many women would vote? Did voters lie when pollsters called? Or were the polls right about Obama leading, proving that debates and campaigning the last weekend really do matter and can sway voters at the last hour?

Regardless of the answers, many analysts urged a postmortem to figure out what the heck happened in New Hampshire.

"It is simply unprecedented for so many polls to have been so wrong," said Gary Langer, the polling director for ABC News, in a memo posted at his Web site. "We need to know why."

Every poll done for the news media in New Hampshire after the Iowa caucuses showed Obama gaining and opening a lead on Clinton.

A McClatchy-MSNBC poll conducted immediately before and after Iowa showed Obama with the support of 33 percent in New Hampshire, Clinton with 31 percent.

Polls conducted after Obama's Iowa win showed him with a bigger lead. One survey for C-SPAN and Reuters showed Obama up 42 percent to 29 percent over Clinton. Six public polls for news media and universities showed him with an average lead of 8.3 percentage points.

None showed Clinton close, let alone ahead. Yet she beat Obama by 39 percent to 36 percent.

SO WHAT HAPPENED?

One possibility widely mentioned yesterday was that white New Hampshire voters might have lied to pollsters, expressing support for black Obama, then voting against him once they were in the privacy of the polling booth.

That's happened before, and it's noteworthy that there was no big discrepancy on the Republican side, where all top candidates were white.

"There will be a lot of claims about what happened, about respondents who reputedly lied, about alleged difficulties polling in biracial contests," Langer said. "That may be so. It also may be a smokescreen, a convenient foil for pollsters who'd rather fault their respondents than own up to other possibilities — such as their own failings in sampling and 'likely voter' modeling."

One possible reason the polls were so far off was that pollsters miscalculated when they screened those who answered their phones to find "likely voters."

Another is that the timing of the polls missed a late surge of support for Clinton, particularly among women, influenced by a debate Saturday, Sunday talk shows, round-the-clock campaigning and an emotional response from Clinton on Monday to the stress of the campaign.

"Timing is half of everything," said Brad Coker, the managing partner of Mason-Dixon Polling & Research, which conducts polls for McClatchy and MSNBC.

In Iowa, a McClatchy-MSNBC poll conducted Dec. 26-28 found Republican Mitt Romney ahead of Mike Huckabee, but the final voting results last Thursday were the reverse.

Coker noted that two other polls taken at the same time produced similar results.

A survey by the American Research Group conducted Dec. 26-28 found Romney with 32 percent and Huckabee at 23 percent. A Strategic Vision poll Dec. 26-27 showed Huckabee with 29 percent, Romney with 27 percent. Both showed Romney with more support than he'd had before Christmas and Huckabee with less.

The Des Moines Register poll took a different snapshot over a slightly different period, Dec. 27-30. It had a larger sample, 800 likely voters in each party versus 400, which cut the error margin from 5 percentage points to 3.5.

The Register poll also assumed a greater turnout by first-time caucus attendees. Coker said he based his assumed turnout on historic averages. He said he thought that the McClatchy-MSNBC poll was accurate at the time, and that circumstances changed to benefit Huckabee.

The key, he said, was Huckabee's decision Dec. 31 to pull a negative ad he'd planned against Romney.

All polls, of course, are just numbers. How they're read depends in large part on how the news media portray them.

Another thing to look for in polls is the margin of error.

That means that 95 percent of the time, any number in a poll could be higher or lower by as much as the margin of error. It's a matter of statistical probability.

The other 5 percent of the time? They're flat-out wrong.