Kennedy Jr is the third of Bobby Kennedy’s 11 children, and at 53 is best known as an environmental activist. He has never previously run for office, but he told Oprah Winfrey in an interview to be published next month in O, the chat show presenter’s magazine: “If Hillary left the Senate, I might run for that seat.”

It was not the first time that Kennedy Jr had hinted that he might enter politics, but Democratic sources saw his remarks as a blunt warning to possible rivals.

Despite his name and his family’s still formidable political clout, Kennedy Jr was long considered an unlikely contender for the high office occupied by so many other members of his family. His father was senator for New York from 1964 until he was shot dead in California in 1968; his uncle Jack was a senator before becoming president and his uncle Ted remains a senator for Massachusetts.

Yet Kennedy Jr’s career prospects were clouded by an arrest in 1983 for heroin possession after he was found unconscious on the floor of a South Dakota airport toilet with a needle stuck in his arm.

After a stint in rehab Kennedy Jr began volunteer work for the Hudson River Foundation, an environmental group fighting pollution of New York’s most important river.

The experience changed his life and he was later offered a job as lawyer to the foundation. That work turned into a lifelong commitment to the Hudson and other rivers.

In recent years Kennedy Jr has become a leading critic of President George W Bush’s environmental policies and an advocate of urgent measures to combat global warming. He wants to run for office, he said, because the Bush administration had made America “the most hated nation on earth”.

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