The Clinton campaign pressed Kennedy for weeks to remain neutral in the Democratic race, but instead he plans to campaign aggressively for Obama, the junior senator from Illinois, The New York Times reported Monday.
Kennedy has worked closely with Clinton, a senator from New York, on health care and other legislation and has had a friendly relationship with her and her husband, former President Bill Clinton, the newspaper reported.
Those close to Kennedy say he was drawn to Obama's inspirational tone.
In an op-ed published Sunday in The New York Times, she said that she had never seen a president who inspired her the way her father had inspired others.
"But for the first time I believe I have found the man who could be that president -- not just for me, but for a new generation of Americans," she wrote of Obama.
But not all of the Kennedy family is backing Obama. Former Maryland Lt. Gov. Kathleen Kennedy Townsend has endorsed Clinton.
"I respect Caroline and Teddy's decision, but I have made a different choice," she said.