They are elusive. They can upend campaigns. They are the great swath of voters who decide just days before or even moments before they actually vote.
All of the polls out in recent weeks, whether they are state polls or national polls, suggest the same thing — that many voters have not made choices and those who have are not fully committed to them.
Campaign operatives and political scientists have long been trying to figure out what besides indecision sets apart late deciding voters and what in the end tips the balance and makes them vote for one candidate over the other.
Historically, primaries have more late deciding voters than do general elections. That may be because, in primaries, voters are choosing between candidates in their own party rather than choosing between candidates of different parties. Party loyalty, then, plays no role.
In 2004, for example, in the New Hampshire Democratic primary, more than half of the voters, 54 percent, waited until the last week to decide whom to vote for. Most voted for John Kerry, a neighboring son whose close proximity to the state made him a known entity before the campaign had even begun.
By the time November came, only 10 percent of voters nationally waited until the last week to decide and they split their votes between Mr. Kerry and George W. Bush, who had already been president for four years and was well known to voters.
“In general, party is the great shortcut for people,” said Marion R. Just, a professor of political science at Wellesley College. “Most people have some preference between the two parties. It may not be strong, but they agree in general with the approach of one party.”
Campaigns struggle against another unknown, voters whose opinion is fungible, who say they are going to vote for one candidate but also say they might change their mind and vote for another. In primaries, candidates from the same party often have similar views on the issues. Voters are less likely to be influenced by a difference in position on issues than by something more ephemeral.
This is particularly true in this year’s campaign among the Democratic candidates, whose views on the issues that voters care most about, the economy and the war in Iraq, are similar. Republican voters are more likely to switch between candidates this year.
In the most recent New York Times/CBS News Poll, taken nationally a few days after Hillary Clinton and John McCain won their respective primaries in New Hampshire, 72 percent of Republican primary voters said they might still change their minds, as did 43 percent of Democratic primary voters. Contributing to the Republicans’ shifting loyalties is the low level of enthusiasm for their field of candidates.
The recent contests provide little insight into what characteristics set late deciders apart. They are just as likely to be men as women, rich as poor, college graduates as high school graduates. They are no more likely to be liberal than moderate or conservative. There are anomalies — in some states, Republican women were more likely to decide late than Republican men — but no pattern.
But the caucus and primary polls taken by Edison/Mitofsky International for the National Election Pool suggest that more than 4 in 10 voters have decided who to vote for in the final week before the contests. In the Iowa Democratic caucus, 41 percent decided late; in the New Hampshire Democratic primary, 49 percent decided at the last minute; in the Michigan Republican primary, 46 percent decided late.
All of which suggests any state’s primary is in play until the waning moments.
Dr. Just suggests that late deciding voters may actually have a preference but are looking and waiting for something to validate the choice they have already made.
“To a great extent people are trying to come up with answers that correlate with their preferences,” she said. “They are just waiting for it to happen.”