Among the notable examples are Nelson Mandela in 1990 and 1994 and Liberian president Ellen Johnson Sirleaf in 2006.

CNN: What you might not know about the State of Union

More noteworthy are the six Keystone converts, which include South Dakota's Tim Johnson, Virginia's Mark Warner, Colorado's Michael Bennet, Florida's Bill Nelson and Delaware's Chris Coons and Tom Carper.

WSJ: Review & Outlook: The Keystone Converts

Included, along with Mary Landrieu, Claire McCaskill , Joe Manchin, Ben Nelson and Debbie Stabenow, are Sherrod Brown (Ohio), Bob Casey (Pennsylvania), Tim Johnson (South Dakota), Tom Carper (Delaware), Kent Conrad (North Dakota), Amy Klobuchar (Minnesota), Jim Webb (Virginia), Carl Levin (Michigan) and John Rockefeller (West Virginia).

FORBES: Reining In The EPA's Reign Of Error