The emails were released on Friday on Twitter by WikiLeaks, which linked readers to a Web page inviting them to search the DNC email database. A search box sits beneath a one paragraph introduction:

“Today, Friday 22 July 2016 at 10.30am EDT, WikiLeaks releases 19,252 emails and 8,034 attachments from the top of the US Democratic National Committee – part one of our new Hillary Leaks series,” the introduction says. “The leaks come from the accounts of seven key figures in the DNC,” including Communications Director Luis Miranda (10770 emails), National Finance Director Jordon Kaplan (3797 emails), Finance Chief of Staff Scott Comer ...” and others. The newly released emails cover the period from January 2015 to May 25, 2016.

Friday’s document dump follows a report last month by The Washington Post that Russian government hackers had penetrated the computer network of the Democratic National Committee, gaining access to an entire database of opposition research, among other material.

DNC and Clinton campaign officials did not respond to requests for comment on Friday as reporters and unnerved campaign staff tried to assess the damage caused by the release, which comes just as the party holds a nominating convention in Philadelphia designed to project unity after a bitter primary season. Some of the emails could open some old wounds and impede that process.

One email written May 5 to Miranda from another party official suggests that the party could help Clinton by raising questions about Sanders’s faith. The email seems to indicate a clear preference among DNC officials for a Clinton primary win.

Another, from a lawyer for the Clinton campaign, suggests a response the DNC should use to refute claims by Sanders that the Clinton campaign was improperly using a joint fundraising committee with the party to raise money that provided benefits to Clinton during the primary season. “The DNC should push back DIRECTLY at Sanders and say that what he is saying is false and harmful the the [sic] Democratic party,” attorney Marc Elias wrote in a note to Miranda on May 3. Elias did not respond to a request for comment late on Friday.

It was well known that there had been friction between the Sanders campaign and the supposedly impartial party apparatus.

But the emails detail exactly how much bitterness enveloped that relationship as Sanders emerged as a real threat to Clinton.

One potential complication is that Sanders’s supporters are crucial to Democratic hopes of retaining the White House in the fall. They bring to the contest both passion and a potentially vast donor base.

The cache of emails also includes communications with journalists and discussions of news organisations, and the emails provide a new perspective on the deference shown to major donors – and the efforts to carefully calibrate rewards based on a contributor’s financial generosity.

In one exchange from May, Mid-Atlantic Finance Director Alexandra Shapiro and National Finance Director Jordan Kaplan argued over which big giver deserved to sit next to President Barack Obama at a DNC event.

Kaplan directed Shapiro to put New York philanthropist Philip Munger in the prime spot, switching out Maryland ophthalmologist Sreedhar Potarazu. He noted that Munger was one of the largest donors to Organising for America, a charity that advocates Obama’s policies. “It would be nice to take care of him from the DNC side,” Kaplan wrote.

Shapiro resisted, saying Munger had only given US$100,600 to the party, while the Potarazu family had contributed US$332,250.

In one email attachment from Erik Stowe, the finance director for Northern California, to Tammy Paster, a fundraising consultant, he lists the benefits given to different tiers of donors to the Democratic National Convention, which starts next week in Philadelphia. The tiers range from a direct donation of US$66,800 to one of US$467,600 to the DNC. The documents also show party officials discussing how to reward people who bundle between US$250,000 to US$1.25 million.

A document titled “2016 Convention Packages” shows that top-tier donors will be treated well in Philadelphia. They will receive priority booking at a premier hotel, free tickets to major convention events and six tickets to an “exclusive VIP party,” according to the document.