Mrs. Clinton has vowed that barring any threats to national security, she would open up government files on the subject, a shift from President Obama, who typically dismisses the topic as a joke. Her position has elated U.F.O. enthusiasts, who have declared Mrs. Clinton the first “E.T. candidate.” “Hillary has embraced this issue with an absolutely unprecedented level of interest in American politics,” said Joseph G. Buchman, who has spent decades calling for government transparency about extraterrestrials. Mrs. Clinton, a cautious candidate who often bemoans being the subject of Republican conspiracy theories, has shown surprising ease plunging into the discussion of the possibility of extraterrestrial beings. She has said in recent interviews that as president she would release information about Area 51, the remote Air Force base in Nevada believed by some to be a secret hub where the government stores classified information about aliens and U.F.O.s.