President Barack Obama urged Americans Tuesday to follow the example of Sens. Orrin Hatch and Ted Kennedy — whom he praised as a radiant example of people who overcome differences to serve their country together in friendship.

That came as Obama signed a bill they sponsored, the Edward M. Kennedy Serve America Act. It authorizes spending $5.7 billion to expand or create a variety of national service programs, including tripling the size of AmeriCorps from 75,000 positions to 250,000. It has been a sort of going-away honor for Kennedy, who is battling brain cancer.

Obama said, "More than anyone else, the new era of service we enter in today has been made possible by the unlikely friendship between these two men, Orrin Hatch and Ted Kennedy. They may be the odd couple of the Senate. One's a conservative Republican from Utah. The other is, well, Ted Kennedy."

Obama added, "But time and again they've placed partnership over partisanship to advance this nation even in times we were told that wasn't possible" as they have worked together on scores of bills through the years. This may be the last one they will push together.

Obama said, "Sen. Hatch was shaped by his experiences as a young missionary serving others, a period he has called the greatest of his life.

"And last year he approached Sen. Kennedy to share his ideas about service. Out of that conversation came this legislation. And last month at Sen. Hatch's selfless request, the Senate unanimously chose to name this bill after his dear friend Ted," the president said.

Obama used the signing of the bill as an opportunity to challenge Americans to give more service.

"A week from tomorrow marks the 100th day of my administration. In those next eight days, I ask every American to make an enduring commitment to serving your community and your country in whatever way you can," he said.

Obama led the way by planting trees at a park later in the day along with his wife, Vice President Joe Biden, former President Bill Clinton and others. "It's as simple as that. All that's required on your part is a willingness to make a difference," he said.

Obama added, "We need your service, right now at this moment in history. I'm not going to tell you what your role should be. That's for you to discover. But I'm asking you to stand up and play your part. I'm asking you to help change history's course, put your shoulder up against the wheel."

He added, "If you do, I promise you your life will be richer, our country will be stronger, and someday years from now you may remember as the moment when your own story and the American story converged."