In particular, Democrats underperformed among male voters, non-college voters, irregular voters and rural voters, it said.
The report was released less than six months before November's midterm congressional elections.
In a statement accompanying the release, DNC chairman Ken Martin said it "does not meet my standards and it won't meet your standards," but he said it was being published to restore trust in the party.
Although the autopsy criticises Democrats' focus on "identity politics," it sidesteps some of the most controversial elements of the 2024 campaign.
The report does not address former president Joe Biden's decision to seek re-election, the rushed selection of Harris to replace him on the ticket or the party's acrimonious divide over the war in the Gaza Strip.
The 192-page document includes a disclaimer at the top of each page stating that it "reflects the views of the author, not the DNC," and notes appended throughout highlight inaccuracies as well as conclusions offered without evidence.
The report was written by Paul Rivera, a Democratic consultant, who could not immediately be reached for comment.
It was completed late last year, and some Democrats were angry it was being kept secret.
While Democrats appear well-positioned to make gains in Congress in the November vote given polls indicating a decline in Trump's popularity, they are still searching for a unifying message ahead of the 2028 presidential campaign.
A New York Times/Siena College poll this week found widespread frustration among Democratic voters of all stripes, even as the party appears to have a sizable advantage over Republicans heading into the election, the newspaper reported.
Martin had initially promised to release the report but changed his mind in December, saying he did not want to encourage Democrats to engage in finger-pointing about 2024 rather than focusing on the future.
The turnabout caused some party supporters to question his leadership.
He wrote that he had withheld the report after last November's Democratic victories in Virginia and New Jersey to avoid distraction but acknowledged the decision only created a larger one.
"For that, I sincerely apologise," he said.
Both major parties have in the past commissioned autopsies following losses to explore what lessons should be learned, including interviewing party leaders, activists and donors and analysing spending and messaging.
The report notes that 2024 was quite close, which might convince Democrats they only need to make minor changes.
But that approach is "denialist at its core," according to the report, which said the party "has vacillated between stagnation and retrogression" since Barack Obama's landslide White House win in 2008.
with AP
