STATE COLLEGE -- Sen. Barack Obama courted the younger voters who have been a key constituency for his campaign as he drew cheers yesterday from thousands sprawling across the lawn in front of Penn State University's Old Main.

Under blue skies laced with thin cirrus clouds, the Illinois senator made one more stop in a cross-state bus tour to repeat his promises of change in Washington.

After months of a bruising campaign, he offered praise for the public service of his White House rivals, Democratic Sen. Hillary Rodham Clinton and Republican Sen. John S. McCain, but described both as mired in the partisanship of Washington.

"Her basic argument is we just need to change political parties," Mr. Obama said of the rival that has led in the polls in Pennsylvania, in contrast to Mr. Obama's advantages in the overall race for the Democratic nomination for president.

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"Just because we have a Democrat in the White House doesn't mean that things are going to change," he said.

"We've been talking about energy independence for decades now, and the only thing that's changed is that gas prices are going up and Exxon Mobil is making $11 billion a quarter and you are struggling to fill up your gas tank.

"It's not enough to change political parties in the White House. We've got to change how politics is done in this country, how politics is done in Washington."

At another point in his standard stump speech, Mr. Obama drew applause from the student-dominated crowd as he described his proposal for tuition assistance in return for public service. Mrs. Clinton has offered a broadly similar plan.

Mr. Obama bowed to Nittany Lion sensibilities, mentioning various members of the politically diverse Paterno family.

"I am honored to have had a chance to say hello to Jay Paterno. Sue Paterno [Joe's wife] is here today," he said from his perch beneath the Corinthian columns of Old Main. "I wanted to say I just talked to your husband and we decided after I get this thing settled, I'm going to have to come back and watch a football game."

Jay Paterno, Penn State's quarterbacks coach, is an Obama supporter. Joe, his legendary father, is a Republican. Mrs. Clinton's father and brother both played football at Penn State.

"I'm grateful to Jay for his wonderful support and to Sue for being here today," Mr. Obama said.

Mr. Obama has traversed what polls suggest is some politically challenging terrain in the first days of his Pennsylvania tour, but the crowd's response and the data from earlier primaries suggested that he was in Obama country yesterday. Older voters have been consistent allies of Mrs. Clinton while younger voters have been a reliable asset for the Illinois senator.

In Pittsburgh, Greensburg and Altoona, Mr. Obama campaigned in communities with higher percentages of older voters than this college town. According to an analysis of exit poll data by the Pew Research Center, younger voters have not only favored Mr. Obama, but they have formed a larger proportion of the overall electorate than in the past, an increase even more noteworthy in that overall turnout has soared in the Democratic contests.

This year, young voters have been a larger slice of a larger pie. In the Super Tuesday contests, voters younger than 30 constituted a higher share of the total Democratic electorate across the boards.

"In all of the 2008 contests for which exit poll data are available, young people have constituted an average [median] of 14 percent of Democratic primary voters, up from a median of 9 percent in the set of comparable contests in 2004," Scott Keeter, director of survey research for Pew, wrote in an analysis earlier this month.

The largest share of that increased turnout went to Mr. Obama in each of those states except California, Arkansas and Massachusetts.

Mrs. Clinton returns to Pennsylvania today for a three-day swing that will bring her to Pittsburgh on Wednesday. The New York senator will be in Harrisburg and Bucks County today, in Philadelphia, Scranton and Erie tomorrow, and in Pittsburgh discussing economic issues Wednesday.

After a morning game of basketball with Sen. Bob Casey yesterday, Mr. Obama visited the university's School of Agriculture dairy complex.

"How are you doing, buddy?" the practiced campaigner said as he schmoozed with one of the cows.

Later, as he was feeding a calf with a bottle of milk, he ordered up a photograph to impress his 9- and 6-year-old daughters.

"Every day they say, 'What did you do today?' 'Well, I gave a speech.' 'Boring.' They are not interested in my work generally. So I can prove to them once in a while."After his Penn State rally, Mr. Obama headed to Harrisburg for an evening town meeting. Today, his bus heads to Lancaster and Allentown.