Armed with the most storied surname in U.S. politics, Robert F. Kennedy's wildcard shot for the presidency holds a clear and present danger for Joe Biden's hopes of a second term in the White House.

The environmental lawyer and conspiracy theorist is boasting double-digit support, and polling suggests that independent candidate "RFK Jr" is hurting the president more than Republican challenger Donald Trump.

Democrats have learned to fear long-shot outsiders after George W. Bush and Donald Trump won tight elections in 2000 and 2016, buoyed by Green Party candidates leeching votes from Al Gore and Hillary Clinton.