The Democratic National Committee reported the organization was hacked again by Russian agents who it says are trying to influence the U.S. presidential election.
A link to the documents was posted on WikiLeaks' Twitter account on September 13 and attributed to a hacker named Guccifer 2.0.
"There's one person who stands to benefit from these criminal acts and that's [Republican presidential nominee] Donald Trump," the committee's interim Chairwoman Donna Brazile said.
"Not only has Trump embraced [Russian President Vladimir] Putin, he publicly encouraged further Russian espionage to help his campaign," she said.
The committee reported a similar leak in July in an incident that roiled the Democratic presidential nominating convention and prompted the resignation of its chairman, Debbie Wasserman Shultz.
Trump afterward invited Russia to dig up more e-mails that would benefit his campaign, although he said later he was being sarcastic.
"We have been anticipating that an additional batch of documents stolen by Russian agents would be released," said Brazile.
Democratic sources said the party and its presidential candidate, Hillary Clinton, are deeply concerned about possible publication by WikiLeaks or other hackers of a new torrent of potentially embarrassing party information ahead of the November 8 election.