WASHINGTON -- Hillary Clinton led Donald Trump by seven percentage points in a national poll of likely voters released Monday.

Clinton polled 49 percent to Trump's 42 percent, according to the Monmouth University survey. In a Monmouth poll released earlier this month after both national political conventions, Clinton had a 54 percent to 40 percent edge.

"The margin has narrowed since her post-convention bounce, but Clinton is holding onto an underlying advantage over Trump among key voting blocs," said Patrick Murray, director of the Monmouth polling institute.

Statistics expert Nate Silver's fivethirtyeight.com website now gives Clinton a 79.5 percent chance of winning the White House in November, down from 89.2 percent two weeks ago.

She has held in all national opinion polls since the conventions, and currently has a 6-point advantage over Trump in the Real Clear Politics average.

Here are three takeaways from the poll:

1. Clinton remains historically unpopular -- and Trump's still worse

Clinton was viewed unfavorably by 51 percent of registered voters and favorably by just 34 percent. Trump's negative rating is 57 percent and positive rating is 26 percent.

More than one-third of the voters, 35 percent, did not have a favorable opinion of either nominee. In polls dating back to 1984, the previous high among voters who had nothing positive to say about either major party candidate was 9 percent.

"This is truly extraordinary," Murray said. "It seems like a significant number of voters are backing a presidential candidate about whom they cannot say anything positive."

2. The Clinton Foundation is hurting Clinton.

A majority of registered voters, 54 percent, said they believed donors to the Clinton Foundation, started by Clinton's husband, former President Bill Clinton, got special treatment in terms of being able to meet with State Department officials.

Just slightly more than one-fourth, 26 percent, said there was nothing unusual.

3. Voters say Trump does have something to hide.

Trump is the only presidential candidate in the modern era to refuse to release his income tax returns, and 52 percent of registered voters say it was because he wanted to keep information from the electorate. Just 24 percent believed Trump's explanation that he cannot release his returns because he is being audited.

More than 7 in 10 voters, 73 percent, were aware that Trump has not released his returns, while 56 percent knew that Clinton already has made hers public. Meanwhile, 62 percent said it was important that candidates release their returns.

The survey of 802 registered voters was conducted Aug. 25-28 and had a margin of error of 3.5 percentage points. The subset of 689 likely voters had a margin of error of 3.7 percentage points.

Jonathan D. Salant may be reached at jsalant@njadvancemedia.com. Follow him on Twitter @JDSalant. Find NJ.com Politics on Facebook