When President-elect Donald J. Trump was declared the victor in this year’s presidential election on Wednesday, the initial vote counts revealed a notable drop of millions of Democratic votes compared to the 2020 results.

For some Republicans, this decrease was seen as proof that the 2020 election had been fraudulent, suggesting that Democrats had somehow manufactured millions of votes in that year — despite repeated validations from election officials, state audits, and courts confirming the integrity of the results.

The shortfall also sparked concerns on the political left, though for different reasons. Numerous social media posts urged Vice President Kamala Harris to refrain from conceding due to suspicions that millions of votes might be “missing” this time around — despite assurances from federal agencies that the election was secure and without interference.

It’s unusual for opposing political groups to create conspiracy theories from the same data, yet supporters of both candidates have fixated on the Democrats’ lower turnout as a key narrative. This online debate underscores how election fraud theories appeal to partisans across the political spectrum, especially those on the losing side, and how social media can amplify these ideas, even when there is clear evidence that the concerns are baseless.

While election officials managed various Election Day challenges, Jen Easterly, director of the Cybersecurity and Infrastructure Security Agency, confirmed there was “no evidence of any malicious activity that affected the security or integrity of our election infrastructure.”

As ballot counting continued in the following days, estimates from The Associated Press indicate that about 157.6 million votes were cast this year, roughly 700,000 fewer than in 2020. The margin between Mr. Trump and Ms. Harris has narrowed as votes from left-leaning states like California, Washington, and Oregon are processed, given the extended time needed for counting mail-in ballots.

The right-wing version of the conspiracy theory gained traction with prominent conservative commentators, while the left-wing variant spread among less widely known voices.

“I hadn’t thought of it this way before, but when he lost in 2020, I kind of felt something was off,” said Tucker Carlson, a Fox News host turned online personality, during an election night livestream that garnered three million views. “Now I’m certain it was rigged.”

Another widely shared post on X by a far-right account with over three million followers featured a chart comparing party turnout, supposedly showing a dramatic increase in Democratic votes in 2020. However, the chart used a distorted axis and preliminary data to exaggerate the difference, with nearly two million views.

Left-wing theories mostly circulated among less prominent figures than Carlson, yet they also garnered millions of views on X, according to a Cyabra report, a company monitoring disinformation. Cyabra reported that posts from accounts with minimal followers managed to amass millions of views by spreading conspiracy theories, which the company believes “damaged trust in the 2024 election and democratic institutions.”

On the left, allegations often centered on Russian interference or the multiple bomb threats reported at polling sites in swing states. Some posts also echoed arguments made by Trump supporters after the 2020 election, claiming that Ms. Harris’s large crowds reflected greater Democratic enthusiasm. However, experts caution that crowd sizes are not a reliable indicator of Election Day turnout.

This week, the hashtag #DoNotConcedeKamala trended on X, appearing in over 650,813 posts by Friday, as reported by NewsGuard, a company tracking misinformation. On Wednesday morning alone, NewsGuard noted more than 30,000 instances of the hashtag alongside terms like “rigged,” “fraud,” or “stolen.”

“If anyone has the means to fund a massive election fraud scheme, it’s Elon Musk,” wrote Dean Obeidallah, host on SiriusXM’s Progress channel, to his 50,000 followers on Threads. He urged the Department of Homeland Security to investigate the results to “address concerns that Trump and Elon Musk may have conspired to rig the election.” Mr. Musk, the owner of X, has been a vocal supporter of Trump. Mr. Obeidallah declined to comment.

So far, disinformation researchers’ fears that this uproar could spark a larger movement have been mitigated by Democratic leaders, who broadly endorsed the election results — a stance that Mr. Trump and his supporters did not take in 2020.

Ms. Harris conceded the race on Wednesday, and House Minority Leader Hakeem Jeffries affirmed that Democrats would respect the outcome, emphasizing that they “stand for free and fair elections.”ions.”