Twitter on Tuesday slapped a fact-check label on President Trump’s tweets for the first time, a response to long-standing criticism that the company is too hands-off when it comes to policing misinformation and falsehoods from world leaders.

The move, which escalates tensions between Washington and Silicon Valley in an election year, was made in response to two Trump tweets over the past 24 hours. The tweets falsely claimed that mail-in ballots are fraudulent. Twitter’s label says, “Get the facts about mail-in ballots,” and redirects users to news articles about Trump’s unsubstantiated claim.

The tweets, said Twitter spokeswoman Katie Rosborough, “contain potentially misleading information about voting processes and have been labeled to provide additional context around mail-in ballots.”

The label directs users to articles by CNN, The Washington Post and the Hill, along with selections from the articles and a page summarizing the findings of fact-checkers.

Twitter’s action quickly drew backlash from Trump and his supporters. Twitter “is now interfering in the 2020 Presidential Election,” the president tweeted. “They are saying my statement on Mail-In Ballots, which will lead to massive corruption and fraud, is incorrect, based on fact-checking by Fake News CNN and the Amazon Washington Post.”

Early Wednesday, Trump railed on social media and repeated his mail-in voting claims. “Republicans feel that Social Media Platforms totally silence conservatives voices. We will strongly regulate, or close them down, before we can ever allow this to happen. We saw what they attempted to do, and failed, in 2016. We can’t let a more sophisticated version of that....” he tweeted.

“Just like we can’t let large scale Mail-In Ballots take root in our Country. It would be a free for all on cheating, forgery and the theft of Ballots. Whoever cheated the most would win. Likewise, Social Media. Clean up your act, NOW!!!!”

For its 14-year existence, Twitter has allowed misinformation by world leaders and everyday citizens to spread virtually unchecked. Its leaders have long said users would engage in debate on the platform and correct false information on their own.

But Trump has made many false claims on social media, particularly on his preferred medium of Twitter, and has also attacked people in ways that critics have argued could violate company policies on harassment and bullying.

For example, Twitter faced a barrage of criticism earlier Tuesday over another set of Trump tweets. The widower of a former staffer to then-Rep. Joe Scarborough asked Twitter chief executive Jack Dorsey to delete tweets by Trump furthering a baseless conspiracy theory about the staffer’s wife’s death. Those tweets are still up, a reflection of social media companies’ approach to policing content that can appear inconsistent even as they have stepped up their enforcement.

Twitter is debating whether to take action on the Scarborough tweets, said a person familiar with the discussions who spoke on the condition of anonymity because he was not authorized to speak publicly.

Its much larger rival Facebook, by contrast, launched a fact-checking program several years ago. Facebook funds an army of third-party fact-checkers to investigate content, which then gets labeled on the site and demoted in its reach. However, Trump posted the same content about mail-in ballots on Facebook.

Facebook said it didn’t plan to label or remove the post. “We believe that people should be able to have a robust debate about the electoral process, which is why we have crafted our policies to focus on misrepresentations that would interfere with the vote,” Facebook spokesperson Andy Stone said.

Twitter, which has roughly 330 million users compared to Facebook’s 2.6 billion, has not had the resources or the institutional will to engage fact-checkers.

But Twitter has changed its approach during the pandemic. In March, the company revised its terms of service to say it would remove posts by anyone, even world leaders, if such posts went “against guidance from authoritative sources of global and public health information.” That includes comments claiming social distancing is ineffective or essential oils can be used to cure the disease, for example.

Soon after, for the first time, Twitter applied the policy to world leaders, removing tweets by Brazilian President Jair Bolsonaro and Venezuelan President Nicolás Maduro, arguing that the tweets about breaking social distancing orders and touting false cures had such potential for harm that labeling them would be insufficient.

In March, Twitter labeled a manipulated video of presumptive Democratic nominee Joe Biden that was retweeted by Trump. That same month, Facebook took down a misleading ad about the U.S. census, one of two times that Facebook has taken action against the Trump campaign.

Then earlier this month, Twitter rolled out a policy saying that it would label or provide warning messages about coronavirus-related misinformation, even when that information is not a direct contradiction of health authorities and does not violate the company’s policies. The company said at the time that it may expand the labels to other issue areas, such as other types of health-related hoaxes or other situations where there is a risk of harm. Tuesday’s tweets on elections represent an expansion into a new area of election-related misinformation.

“It’s clear that social networking sites have a critical role to play in disseminating democratic speech and therefore in helping to police the boundaries of that speech," said Joshua Pasek, an associate professor of communication and media at the University of Michigan. "The fact that Twitter felt the need to take this action suggests that there is a point at which the preservation of democracy in their view overshadows the importance of remaining non-partisan.”

As a matter of policy, Twitter and other tech companies hold world leaders to different standards than everyday users. The content of world leaders is kept up by Facebook, Twitter and Google’s YouTube, even when it violates company policies, a practice known as the “newsworthiness exemption.”

That policy has long been subject to criticism because comments by world leaders can have massive impacts on people’s behavior and the potential to cause harm. Trump’s recent promotion of the drug hydroxychloroquine as an experimental treatment for the novel coronavirus, for example, caused prescriptions and sales to soar.

If Trump had instructed people to take the drug outright, the statement would probably have been taken down by both Facebook and Twitter, according to people who work there who spoke on the condition of anonymity because they were not authorized to speak publicly. Instead the president walked a fine line, promoting the benefits of the drug and saying he was taking it himself.

The World Health Organization has halted studies of the drug out of concern that it causes more harm than good.

Cat Zakrzewski contributed to this report.