Senator Edwards’s and Senator Obama’s videos, both of which run longer than an hour, have not been up as long and have still fewer viewers. The biggest draw has turned out to be Representative Ron Paul, whose July visit has been viewed, or at least started, more than 350,000 times.

For perspective, consider the numbers that short-form videos of a less serious nature draw. Search for “Barack Obama” on YouTube and you will find that the most-viewed video is titled “I Got a Crush ... on Obama.” It lacks narrative, content and anything other than a young woman with large breasts lip-synching, but it has tallied more than four million views. The most-viewed video that turns up for a “Hillary Clinton” search is “Vote Different,” a dark parody of Apple’s “1984” commercial that portrays the senator most unflatteringly, as a giant TV image that is shattered. It is also approaching four million views.

YouTube has a separate section, “YouChoose ’08,” that gives each candidate a protected space for more serious discourse, similar to the way the broadcast networks give Sunday mornings over to civic uplift. YouChoose also provides access to last Wednesday’s CNN/YouTube debate with the Republican candidates, and the earlier one in July with the Democrats.

Professor Jamieson credits YouTube with broadening the range of questions in the debates, making them more memorable by having users submit the questions in the form of personal videos, and making everything searchable afterward. In the past, she said, “if you missed a debate, you missed it.”

The ability to select for playback any question in the debate and the candidates’ responses provides easy, precise access to the contents, sliced and diced, that was never possible before. But it also contributes to a shortening of our collective attention span.

THIS is hardly new — we’ve already come a long way from the Lincoln-Douglas debates of 1858 for a Senate seat, which held the audience rapt, on one occasion, for three hours — then everyone dispersed for dinner and came back for the four-hour rebuttal. The contrast with the public’s attenuated attention in the age of television, which Neil Postman pointed out in his 1985 book “Amusing Ourselves to Death: Public Discourse in the Age of Show Business,” was great. The contrast is all the greater today, with the advent of the short, nonlinear clips of YouTube.

It is easy to forget that this is YouTube’s first presidential campaign: the company was founded in only 2005 and acquired by Google in 2006. By the time the next campaign cycle rolls around in 2011, YouTube’s influence on the culture may be so complete that a 45-minute linear video of a question-answer session will seem to most people to be about 43 minutes too long.

A midcampaign trek to Google headquarters in Silicon Valley may soon seem no less quaint than one to a G.M. plant in Flint, Mich. The candidates need not seek out the cameras — from now on, the cameras will always find them.