The hacking spree that upended the presidential election wasn’t limited to Democratic National Committee memos and Clinton-aide emails posted on websites. The hacker also privately sent Democratic voter-turnout analyses to a Republican political operative in Florida named Aaron Nevins.

Learning that hacker “Guccifer 2.0” had tapped into a Democratic committee that helps House candidates, Mr. Nevins wrote to the hacker to say: “Feel free to send any Florida based information.”

Ten days later, Mr. Nevins received 2.5 gigabytes of Democratic Congressional Campaign Committee documents, some of which he posted on a blog called HelloFLA.com that he ran using a pseudonym.

Soon after, the hacker sent a link to the blog article to Roger Stone, a longtime informal adviser to then-candidate Donald Trump, along with Mr. Nevins’ analysis of the hacked data.