The Bloomberg News poll conducted by Selzer and Co. found that Clinton's popularity has dropped from 70 percent in December to 58 percent.

The percentage of people who have an unfavorable view of Clinton has increased, the poll also found. In December, 11 percent of those surveyed said they had an unfavorable view of Clinton. Now, 22 percent say they have an unfavorable view of the former secretary of State.

Clinton is regularly mentioned as a strong presidential candidate in 2016. David Redlawsk, a political science professor at Rutgers University, told Bloomberg that since Clinton is often mentioned as the strongest potential candidate in the 2016 president race, she's the target of Republican criticism, which weighs down her favorability.

"The Clintons have been favorite targets of the Republican right for a very, very long time, but it is also because of her presumed front-runner status," Redlawsk said. "In some ways, she's even more of a target" than the president.

The poll was conducted from May 31 to June 3 among 1,002 adults. It had a margin of error of plus or minus 3.1 percentage points.