Use Discover Advanced and select Subject Terms - SU from the dropdown.

Use "subject begins with" for Library of Congress Subject Headings such as these...

Presidents - United States - Election - [year]

For candidates, search by subject with the last name first:

Reagan, Ronald
Gore, Al, 1948-
Humphrey, Hubert H. (Hubert Horatio), 1911-1978
Debs, Eugene V. (Eugene Victor), 1855-1926