May 25 (Bloomberg) -- Barack Obama, standing in for Senator Edward M. Kennedy as commencement speaker at Wesleyan University, invoked the Kennedy family's legacy of public service and challenged students to look beyond material gains and work for our ``collective salvation.''

``No one is forcing you to care,'' Obama said. ``You can take your diploma, walk off this stage and chase only after the big house and the nice suits and all the other things that our money culture says you should buy. But I hope you don't.''

With a commanding lead in the race for the Democratic presidential nomination, Obama said that if he is elected he will call upon the students and the nation to ``be unified in service to a greater good. I intend to make it a cause of my presidency.''

Wesleyan officials estimated the crowd at 25,000, including those who viewed the speech on closed-circuit television in rooms around the campus set up to handle the overflow. Last year, 8,000 attended the college's 175th anniversary celebration.

Obama pinch-hit for Kennedy at the Middletown, Connecticut, school after the 76-year-old Democratic Party lion was diagnosed with a malignant brain tumor. Kennedy, along with his niece, Caroline Kennedy Schlossberg, endorsed Obama in January, a symbolic ``passing of the torch'' to the Illinois senator.

`Not Done Yet'

``It is rare in this country of ours that a person exists who has touched the lives of nearly every single American without many of us even realizing it,'' Obama said in tribute to his cancer-stricken Senate colleague from Massachusetts. ``And I have a feeling that Ted Kennedy is not done just yet.''

Obama said the students should draw inspiration from Kennedy and devote their energies to the common good.

``It's because you have an obligation to yourself,'' Obama, 46, said. ``Because our individual salvation depends on our collective salvation.''

Among the Wesleyan graduates today is Kennedy's stepdaughter, Caroline Raclin. The commencement also marks the 25th reunion for Kennedy's son, Edward Kennedy Jr., who graduated from the school in 1983.

Recalling his time as a community organizer in Chicago, Obama said there are many ways the students can help others.

``At a time of war, we need you to work for peace,'' he said. ``At a time of inequality, we need you to work for opportunity. At a time of so much cynicism and so much doubt, we need you to make us believe again.''

`Childhood Adrift'

He reminded the audience he was raised by a single mother and his grandparents, living overseas and in Honolulu, after his Kenyan father left when he was 2-years-old. ``I spent much of my childhood adrift,'' said Obama, who lived in Indonesia during much of his time in elementary school.

After graduating from Columbia University in New York, he said his decision to take a $12,000-a-year job as a community organizer on Chicago's South Side changed his life. ``Through service, I discovered how my own improbable story fit into the larger story of America,'' he said.

Obama also referred to his father's roots by noting that two graduating Wesleyan students plan to go to Kenya to help bring alternative sources of energy to poverty-ridden areas.

He told the graduating class that the world they are entering will need a ``generation of volunteers' to work on renewable energy projects as governments battle global climate change. He also stressed the need for more people to act as academic mentors to children and donate their time to high-need areas such as New Orleans or a local homeless shelter.

Wesleyan Graduates

Today's appearance gives Obama more exposure to a network of wealthy, high-profile Wesleyan graduates, including Sun Microsystems Inc. Chief Executive Officer Jonathan Schwartz, former Southwest Airlines Co. Chairman Herb Kelleher, ``The West Wing'' television show actor Bradley Whitford and New England Patriots head coach Bill Belichick.

Kennedy has served in the Senate since 1962, making him the second-longest-serving current lawmaker in the chamber after Senator Robert Byrd, a West Virginia Democrat.

In 1984 he received an honorary degree from Wesleyan, a school of 2,700 students that is sometimes confused with Wellesley College, New York Senator Hillary Clinton's alma mater, or Ohio Wesleyan University.

Kennedy wasn't present at today's graduation ceremony. His wife, Vicki, did attend.

Wesleyan is tied for 11th place on U.S. News & World Report's ranking of top American liberal arts colleges.

Obama, who graduated from Columbia University in New York and obtained a law degree from Harvard University in Cambridge, Massachusetts, had no previous ties to Wesleyan. Today he received an honorary degree of laws from the school.

To contact the reporter on this story: Kim Chipman in Middletown, Connecticut at kchipman@bloomberg.net.

Last Updated: May 25, 2008 14:21 EDT