At 54 percent, the rating is the lowest since 2008, when Clinton ended her campaign for president. While she was President Obama's secretary of state, more than 60 percent of the public consistently had a favorable view of her.
In February, just after she left the cabinet, 59 percent of the public viewed her favorably.
Clinton has said she will announce later this year if she will run for the Democratic presidential nomination in 2016. She is currently promoting Hard Choices, her book about her time as secretary of state, an effort that looks in many ways like a campaign.
While she was in the cabinet, her favorability rating was as high as 66 percent, and even 41 percent of Republicans took a favorable view in the summer of 2012, shortly before the deadly attack on the U.S. consulate in Benghazi, Libya.
Gallup suggested that the ramped-up Republican investigation into the Benghazi attack, questions about the release of Bowe Bergdahl, the U.S. soldier held by the Taliban, and suggestions that her health might be shaky have damped public enthusiasm for Clinton. But Gallup analysts said the public always tends to be more enthusiastic about those seen as non-political (Clinton as presidential spouse and secretary of state) and those perceived as political (Clinton as a likely presidential candidate).
The poll found that 64 percent of the public currently view President Bill Clinton favorably.
Gallup surveyed 1,027 adults by telephone betweeen June 5 and June 8. The margin of error is 4 percentage points.