The WikiLeaks founder Julian Assange said on Wednesday that he had rebuffed a request for help last year from the head of a data firm that worked for Donald J. Trump and is now facing congressional scrutiny.

On Twitter, Mr. Assange said he had been approached before the 2016 election by Alexander Nix, the chief executive of Cambridge Analytica, which worked for Mr. Trump during the final months of the campaign. Mr. Assange did not disclose what kind of help Mr. Nix sought, only that he had declined the request.

“I can confirm an approach by Cambridge Analytica,” Mr. Assange wrote, “and can confirm that it was rejected by WikiLeaks.”

But The Daily Beast reported on Wednesday that Mr. Nix had emailed Mr. Assange looking for copies of more than 30,000 emails that were deleted from Hillary Clinton’s private server and never publicly released. Mrs. Clinton has said that the emails were personal in nature.

A spokesman for Cambridge Analytica did not respond to requests for comment on Wednesday evening.

It is not clear precisely when the two men corresponded. CNN reported on Wednesday that the emails were exchanged in the summer of 2016. Cambridge Analytica was being paid by a rival campaign — that of Senator Ted Cruz of Texas — through early June, according to Federal Election Commission records. By early summer, Cambridge Analytica had also begun wooing the Trump campaign, which hired the firm in June. The firm’s principal owner is the conservative billionaire Robert Mercer, who backed Mr. Cruz during the campaign before switching his allegiance to Mr. Trump.

It is also unclear why Mr. Nix would have believed that Mr. Assange had copies of the missing emails. Earlier last year, WikiLeaks had posted a searchable database of more than 50,000 emails from Mrs. Clinton’s private server, all of them previously released by the State Department. But Mr. Trump himself seemed eager to find the missing emails: At a campaign rally in July, Mr. Trump publicly asked Russia to obtain the deleted emails.

“Russia, if you’re listening, I hope you’re able to find the 30,000 emails that are missing,” Mr. Trump said.

The communication with Mr. Nix could more closely link the Trump campaign and Mr. Assange, whose website has published thousands of emails stolen from Democratic officials. United States intelligence agencies believe the documents were originally obtained by Russia-linked hackers. Another Trump adviser, the political consultant Roger Stone, has disclosed that he was in touch with Mr. Assange through intermediaries; during the campaign, Mr. Stone occasionally previewed WikiLeaks releases of stolen emails from Democratic officials.

Cambridge Analytica has drawn criticism inside and outside the Republican Party, both for its claim to be able to classify voters by psychology and for exaggerating its role in Mr. Trump’s upset victory. Former Trump campaign officials have said publicly and privately over the last year that Cambridge functioned as one of several data and analytics vendors for the campaign. While the Republican National Committee provided the campaign’s core voter data, those officials said, Cambridge provided personnel to the campaign and helped develop target lists for digital advertising and online fund-raising, among other tasks.

Cambridge Analytica has since provided documents to congressional investigators investigating potential collusion between the Trump campaign and Russian interests. On Tuesday, Brad Parscale, a former senior Trump campaign official who oversaw Mr. Trump’s digital and analytics vendors, testified before the House Intelligence Committee.

On Wednesday, Michael Glassner, executive director for Mr. Trump’s re-election committee, issued a statement that conspicuously did not mention Cambridge Analytica and seemed intended only to distance Mr. Trump from the firm.

“We as a campaign made the choice to rely on the voter data of the Republican National Committee to help elect President Donald J. Trump,” Mr. Glassner said. “Any claims that voter data from any other source played a key role in the victory are false.” He did not mention the exchange between Mr. Nix and Mr. Assange.