HARRISBURG -- A nationwide drive to register voters for the Nov. 4 election, headed by Democratic National Chairman Howard Dean, stopped yesterday at Harrisburg's High School of Science and Technology to drum up support for U.S. Sen. Barack Obama of Illinois, the presumptive Democratic nominee.

Mr. Dean, a former governor of Vermont, said that the voter registration tour began in Republican President George W. Bush's hometown of Crawford, Texas. The idea is to promote grass-roots methods for increasing the number of registered Democrats and giving Mr. Obama a better chance to win in November.

Mr. Dean challenged about 100 members of his audience, most of whom were high school students, to each knock on 25 doors in their own neighborhoods three times between now and Nov. 4. Doing so, he said, would provide the kind of personal interaction that drives politics and wins votes for Mr. Obama.

"This is a very important state for us," said Mr. Dean. "We have to win Pennsylvania."

The Democratic presidential candidate has carried the state each election since 1992.

"Don't underestimate the power you young people have," Mr. Dean said to the many students in the audience.

Mr. Dean also touted some of Mr. Obama's key goals: universal health care, uniting the country and restoring American moral authority abroad.