Democratic presidential nominee Hillary Clinton will release her 2015 tax returns "in the coming days," a source close to the campaign tells CBS News.

The previous year's returns will add to the past eight years of documents published on the Clinton campaign website last summer.

In all, the Clintons have made their tax returns public for every year dating back to 1977.

Clinton's vice presidential candidate Tim Kaine, a Virginia senator, and his wife Anne Holton are set to release the last 10 years of their tax returns as well.

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Donald Trump, the Republican presidential nominee, has yet to publicly disclose any tax returns from recent years, claiming that they are under review by the Internal Revenue Service (IRS).

Despite numerous calls -- and a couple of million-dollar bounties -- to release them, the billionaire, who owns the international development company the Trump Organization, has said that he would only publish them widely once the IRS audit is complete.

Back in May, Paul Manafort, the chairman of the Trump campaign, said he would be "surprised" if the GOP nominee released his tax returns during the presidential campaign.

"I will be surprised if he puts them out," Manafort told the Huffington Post in an interview. "I wouldn't necessarily advise him to. It's not really an issue for the people we are appealing to."

"His tax returns are incredibly complicated," Manafort went on to say. "I wouldn't understand them, so how are the American people going to?"