Hillary Clinton's nearest rival for the Democrat presidential nod, independent Vermont Sen. Bernie Sanders, has doubled his favorable ratings from 12 percent to 24 percent since Gallup began tracking in March.

Amidst a spate of congressional investigations into her use of private email during her stint as secretary of state, Clinton's unfavorable rating rose to 46 percent, her worst score since December 2007. The number of people who view her favorably also dropped from 48 percent to 43 percent in April, Gallup reports.

Sanders has been attracting huge crowds, including the largest to date in Iowa. Earlier in July, over 7,500 people showed up to his scheduled town hall forum in Portland, Maine, forcing a change of venue, the Washington Examiner reported.

His broad appeal is reflected in the Gallup poll: While in March, only 24 percent of the public were familiar with him, that number nearly doubled to 44 percent. Still, 56 percent of the American public are not familiar enough with Sanders to rate him, and barely half the Democrats surveyed know enough about him to form an opinion. By contrast, just 11 percent of Americans say they are too unfamiliar with Hillary Clinton to form an opinion.

Like Clinton, those who view Sanders unfavorably have also risen during this time, from 12 percent to 20 percent.

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