Bill Weld, 73, ran for vice president on the Libertarian Party ticket in 2016 and has been critical of Donald Trump.
A former governor of Massachusetts and federal prosecutor, Weld focuses his campaign on fiscal restraint, free trade and immigration reform. He has also endorsed steps to legalize marijuana nationwide.
Weld served as the Republican governor of Massachusetts from 1991 to 1997. Prior to that, he was U.S. attorney for Massachusetts and assistant U.S. attorney general at the Department of Justice's criminal division. He re-entered private law practice after 1997.