“I do this for a living, and I’d never heard anyone say the servers were in Ukraine,” she said.

Twitter removed a video posted by Mr. Trump that showed a meme of Nickelbacker’s lead singer, edited as an attack on Joe Biden.

In the 27 months between Mr. Trump’s two citations of the CrowdStrike-Ukraine conspiracy theory, it has survived despite many denials from CrowdStrike, the F.B.I. and people directly involved in the investigations. It has survived despite the fact that the D.N.C. put one of its hacked servers on display — not in Ukraine but in its Washington offices beside the filing cabinet pried open in 1972 by the Watergate burglars (and a photo of the two artifacts ran on The Times’s front page). It has survived despite the indictment prepared last year by Robert S. Mueller III, the special counsel, laying out in extraordinary detail the actions of 12 named Russian military intelligence officers who hacked the D.N.C. and other election targets.

The speculation springs from what Mr. Trump has called a “big Dem scam” — the false notion that the F.B.I. never really investigated the D.N.C. hack. In fact, according to people directly involved, CrowdStrike was in regular contact with the bureau in spring 2016 as it examined dozens of servers used by both the D.N.C. and the Democratic Congressional Campaign Committee.

It is true, as Mr. Trump has often tweeted, that F.B.I. agents never took physical possession of the Democrats’ servers. But CrowdStrike supplied the F.B.I. with digital copies of the servers so that the bureau could assess the Russian malware infecting them. The Mueller investigation later confirmed CrowdStrike’s findings.

Still, the president has clung to the theory linking CrowdStrike, Ukraine and the D.N.C. servers despite the repeated efforts of his aides to dissuade him, Thomas Bossert, his former homeland security adviser, said on Sunday on ABC’s “This Week.” “The D.N.C. server and that conspiracy theory has got to go,” he said. “If he continues to focus on that white whale, it’s going to bring him down.”

To go in search of the roots of Mr. Trump’s CrowdStrike-Ukraine conspiracy theory is to travel the internet’s most peculiar provinces and the darkest threads on Twitter and Facebook. On 4chan and pro-Trump spaces on Reddit, on websites like ZeroHedge.com and Washington’s Blog, you can find plenty of speculation about evil manipulation by CrowdStrike and secret maneuvers by Ukrainians — often inflamed by Mr. Trump’s own statements.