DES MOINES  Jason Huffman has lived in Iowa his whole life. Lately he has been watching presidential debates on the Internet, discussing what he sees with friends and relatives. But when fellow Iowans choose among presidential candidates on Thursday night, he will not be able to vote, because he is serving with the National Guard in western Afghanistan.

“Shouldn’t we at least have as much influence in this as any other citizen?” Captain Huffman wrote in an e-mail interview.

He is far from the only Iowan who will not be able to participate. Because the caucuses, held in the early evening, do not allow absentee voting, they tend to leave out nearly entire categories of voters: the infirm, soldiers on active duty, medical personnel who cannot leave their patients, parents who do not have baby sitters, restaurant employees on the dinner shift, and many others who work in retail, at gas stations and in other jobs that require evening duty.

As in years past, voters must present themselves in person, at a specified hour, and stay for as long as two. And if these caucuses are anything like prior ones, only a tiny percentage of Iowans will participate. In 2000, the last year in which both parties held caucuses, 59,000 Democrats and 87,000 Republicans voted, in a state with 2.9 million people. In 2004, when the Republicans did not caucus, 124,000 people turned out for the Democratic caucuses.