rapid rise in the use of cellphones and caller identification technology, along with telemarketing calls that are chasing Americans from their telephones, is making political polling more difficult and increasingly less reliable, pollsters say. A result this Election Day is that it is harder than ever for pollsters to find voters and to get them to say how they intend to vote.

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Pollsters say a problem that they first began noting 10 years ago, as Americans realized that answering machines could be used to screen out unwanted solicitations, is today forcing a re-examination of the methods by which they question voters.

In interviews, several pollsters said they now discussed ways to change how they approached a fundamental procedure in politics that has, over 75 years, moved from the mail to door-to-door canvassing to the telephone.

"At some point, there's going to be a crash between what's happening in the country and what's picked up on the phone," Stanley Greenberg, President Bill Clinton's pollster in the White House, said yesterday.

Whit Ayres, a veteran Republican pollster, said: "I can't fathom 20 years from now the telephone remaining the primary means of data collection. This industry is in a transition from telephone data collection to Internet data collection."

"In the meantime," Mr. Ayres said, with a note of frustration in his voice, "we've got to get people to answer the phone."

Pollsters said the increasing difficulty in reaching people was undercutting their efforts to assemble a pool of voters that was scientifically large enough and diverse enough upon which to draw reliable conclusions. While some pollsters said they could compensate for that by staying in the field longer or calling more people, that kind of effort takes time and costs money, two things that are often in short supply at the end of a campaign.

Pollsters said they had tried to respond to the problem with various time-consuming and costly remedies. They have increased the time they spend in the field, employed teams of specialists to methodically call back numbers that are answered by machines, and mathematically adjusted their findings at the end of the survey period to make up for voters they might have missed.

But several described these as stop-gap measures that have been only partly successful. Pollsters are under intense pressure to move quickly and hold down costs. They are uncomfortable with the kind of statistical adjustments used to compensate for missed or refused calls.

In particular, pollsters said they might be undercounting the growing number of younger voters who only have cellphones, as well as elderly voters who, they said, tend to be especially wary of any call that sounds like a solicitation. Several pollsters said the rise in the number of unlisted telephone numbers was more pronounced in minority and low-income neighborhoods.

In a case that drew much notice over the weekend, two polls trying to measure the Senate contest in Minnesota produced opposite results: one had Walter F. Mondale with a six-point lead, while the other had Norm Coleman with a six-point lead.

Howard Wolfson, the executive director of the Democratic Congressional Campaign Committee, said he had seen instances this year where two polls by different pollsters in the same district had produced findings so different that it was as if they had come from different states.

"There is a lot of evidence that all of this is making our life more difficult and hurting our efforts," Mark A. Schulman, the president of the American Association of Public Opinion Research, said yesterday.

But, Mr. Schulman added, "I'm not ready to run up the white flag and concede defeat."

Matthew Dowd, a Republican pollster who advises the White House, said: "Right now, I'd still make the argument that polling is the best way to find things out. You haven't gotten to a point where you can't trust it, but you have gotten to a point you have to weigh it."

One prominent pollster said the number of telephone calls that were not completed — either because no one answers the telephone or because they answer and refuse to participate — had jumped in recent years, to about 30 percent from 10 percent. The number is even higher in New York and in South Florida.

"Response rates are falling," said Tom W. Smith of the National Opinion Research Center at the University of Chicago. "Either you spend a lot more time and money doing your survey or you end up being stuck with a much lower response rate than is traditionally acceptable."

As a rule, political polls are done by candidates, political committees or news media outlets. There was some debate yesterday which of these conductors of polls would prove to be most affected by mounting difficulties.

The technological burdens facing the pollsters are expanding. Telephone answering machines and caller ID make it easy for potential respondents to screen out calls — something that is more likely in urban and suburban areas, where people are more likely to be the target of aggressive commercial telephone solicitations.

Mr. Smith said that to complicate things for people in his profession, new screening machinery had been developed that can, in theory, identify calls that are being made by a mass dialer or can refuse calls from any unknown number.

Cellphones have posed another complication for pollsters. There is no directory of cellphone numbers, and an increasing number of people use cellphones as their home telephones. Mr. Schulman said federal regulations barred pollsters from calling people on their cellphones without permission, because the recipients of the calls are obliged to pay the cost.

Several pollsters said yesterday that they would prefer, if possible, not to conduct interviews with people on cellphones. These interviews typically take 20 minutes or so, and were intended to be done with people sitting at home rather than chatting on a cellphone from a car or restaurant.

"We haven't come to grips with the cellphone issue yet, I'll be honest with you about that," Mr. Schulman said. "Up to this point, the cellphone has generally been the second phone for the hard-wire phone in the household. In the future, we've got to figure out a strategy to deal with this."