Facebook and Twitter took unusual steps Wednesday to limit readership of an article by the New York Post about alleged e-mails from Democratic presidential nominee Joe Biden’s son, one of the rare occasions they have sanctioned a traditional media outlet.

The social media giants took that action before verifying the contents of the article, in which President Trump’s personal attorney Rudy Giuliani and his former top adviser Stephen Bannon claimed to have obtained and leaked a trove of private materials from Hunter Biden. The leaked documents suggested at one point he gave a Ukrainian executive the “opportunity’’ to meet the former vice president. The Biden campaign said his schedule indicated no such meeting took place.

Facebook preemptively limited the spread of the story while sending it to third-party fact-checkers, a decision the company said it has taken on various occasions but is not the standard process. Twitter allowed the story to surge to a No. 3 trending topic in the United States, although later marked the link as “potentially unsafe’’ and blocked it. It also temporarily locked White House press secretary Kayleigh McEnany’s account, as well as the New York Post’s, adding notices to their tweets saying they violated Twitter’s rules on prohibiting publishing hacked materials. Trump’s campaign account was also temporarily locked.

The moves prompted an outcry from Trump, Republicans, and right-leaning publications, which repeated claims of politically motivated censorship by Silicon Valley giants.

“So terrible that Facebook and Twitter took down the story of ‘Smoking Gun’ emails related to Sleepy Joe Biden and his son, Hunter, in the @NYPost,’’ Trump wrote on Twitter.

But Wednesday’s actions were the result of a year of tech companies’ scenario-planning exercises for the 2020 election, including the possibility of a “hack-and-leak’’ situation of potentially unverified e-mails that could help swing the election.

Four years after Russian operatives exploited tech giants’ services during a presidential contest, the companies’ swift and aggressive steps in responding to the unverified story, and their divergent responses, are a real-time case study in their ability to protect the integrity of an election that has been marred by domestic disinformation and misleading accounts.

Some of the dozens of scenarios Facebook and Twitter prepared for were along the lines of the 2016 campaign, when Russia-tied WikiLeaks dumped the e-mails of Hillary Clinton’s campaign chair John Podesta. Many news organizations at the time covered the e-mail dump without first sufficiently exploring the organization’s political motivations and Russia ties.

Earlier in the day, Facebook spokesman Andy Stone tweeted that the company was “reducing’’ the story’s distribution while it was checked by independent fact-checkers. He pointed to a link on the company website to a year-old policy, which says that if the company has “signals’’ that a piece of content is false, its distribution could be reduced pending fact-checker review, part of an effort to take “faster action’’ to stop viral misinformation.

Google appeared to take a middle-of-the-road approach. A search for “Hunter Biden’’ revealed the link to the original New York Post story, as well as links to stories and comments rebutting it or questioning its provenance.