A pair of NBC News/Marist polls released Sunday indicated that Clinton's favorability rating among voters in key early-voting states dipped.

According to the polls, Clinton trails Vermont Senator Bernie Sanders by nine points in New Hampshire, and her lead over Bernie Sanders in Iowa has shrunk from 24 points in July to 11 points.

A Gallup poll on Friday also showed that Clinton's favorability rating has dropped to an all-time low.

At a press conference in March, Clinton said she had exchanged about 60,000 emails from her private email account during her stint in the Obama administration, among which about half were personal and thus deleted. The Clinton camp turned over the other half, 30,000 emails in total, to the State Department last year.

Clinton's Republican rivals have long claimed that Clinton had deleted certain work-related emails, mainly on the 2012 Benghazi attacks that claimed four American lives, including the US ambassador to Libya, to protect herself.

The controversy around Clinton's email practices again burst into public view earlier August after the inspector general for the US intelligence community revealed that two of the thousands of emails held by Clinton contained top-secret information.

According to a report by The New York Times published Tuesday, a second review of the two emails by US intelligence community concluded that Clinton received as secretary of state on her private email account highly classified information.

However, both the Clinton camp and the US State Department disagreed with the conclusion and argued that agencies within federal government often have different views of what information should be regarded as classified ones.