Hillary Clinton's 13-point lead over Donald Trump has shrunk by 6 percentage points since the beginning of August, according to Monmouth University's latest national survey of likely voters.

The Democratic presidential hopeful leads Trump 46-39 percent among voters likely to participate in the Nov. 8 election, down from 50-37 percent earlier this month. Clinton's lead among registered voters declined by 5 percentage points during the same period.

While Trump's support among self-identified Republicans has decreased by a single percentage point since early August, Clinton has lost 7 percent support — from 92-85 percent — among members of her own party.

The former secretary of state has, however, taken over the lead among independents after trailing closely behind Trump in the previous national survey. She now leads her Republican opponent 37-32 percent among independent voters.

Clinton's overall decline in support appears to be due in part to the latest developments regarding the Clinton Foundation and its questionable dealings with the State Department when she was at its helm.

Fifty-four percent of voters said they are inclined to agree that Clinton Foundation donors were rewarded with special treatment by the former secretary of state, while 26 percent felt she did nothing wrong.

While Clinton faces mounting questions about her and her aide's practice of granting access to foundation donors, Trump's refusal to release his tax returns could be creating problems of his own.

Thirty-one percent of voters said it's important for both candidates to make their tax documents public, and another 52 percent believe the GOP presidential nominee has refused to do so because there is something in his tax returns that could be damaging to his image.

Both Clinton and Trump remain wildly unpopular among voters, but the latest survey highlights just how dissatisfied voters are with their options. At 35 percent, the percentage of voters who could not say whether they had a favorable view of either major party candidate is unprecedented.

For comparison, the percentage of voters who have been unable to give a positive rating of either candidate has varied between 3-9 percent since the 1984 election between Ronald Reagan and Walter Mondale.

"This is truly extraordinary. It seems like a significant number of voters are backing a presidential candidate about whom they cannot say anything positive," Patrick Murray, director of the Monmouth University Polling Institute, said in a statement.

The survey of 802 registered voters was conducted Aug. 25-28, the week after Trump promoted Kellyanne Conway to be his campaign manager and saw the departure of campaign chairman Paul Manafort. Results contain a margin of error plus or minus 3.5 percent.