As Facebook cracked down on disinformation flooding its social media platforms, executives decided to codify a key loophole: Politicians remained free to lie at will — unbound by the rules designed to stop everyday users from peddling viral falsehoods.

This decision, put into place last year, has sparked a sharp backlash this week among Democrats, who complain that it gives President Trump free rein to use major social media platforms as disinformation machines. Sen. Elizabeth Warren (D-Mass.), a leading presidential candidate, made this point in a Facebook ad Thursday in which she joked that the company had endorsed Trump, adding that its policies allow “a candidate to intentionally lie to the American people.”

Warren’s ad was the latest salvo in a growing campaign by Democrats to pressure social media companies to curb Trump’s ability to push demonstrably untrue information on their platforms. Last week, Democrat Joseph Biden asked Facebook to remove a Trump campaign ad that made false claims, prompting the company to refuse on the grounds that political speech is not covered by the expansive fact-checking system it put in place after the 2016 presidential election.

The Democratic complaints have emerged as a counterpoint to long-standing claims by Republicans, including Trump, that social media platforms and their mostly liberal workforces unfairly tilt the playing field against conservatives and their ideas. This argument, pushed in tweets and public statements, has made the social media companies timid in enunciating and enforcing common-sense standards of behavior online, say Democrats, who contend that the dominant force in Silicon Valley is not political liberalism but the quest for market dominance and profits.

“Facebook has not only created a space where we know misinformation has run rampant for a long time, but they’ve always allowed the Trump campaign to take advantage of the platform to spread blatantly false posts and advertising,” said Tara McGowan, the chief executive of ACRONYM, a progressive nonprofit that tracks digital ad spending.

Deception is hardly new to politics, and candidates have run ads inflating their records and trashing their opponents on television and radio for years. But those falsehoods now, in the age of social media, can go viral in a matter of minutes, reaching millions of people around the world.

The pressure to more aggressively police disinformation has left Facebook and its Silicon Valley peers in a precarious position. Democrats and Republicans alike agree that social media sites must be more vigilant to protect civic discourse after Russian agents easily seeded propaganda on the Internet’s most popular platforms during the last presidential campaign. But doing so would require Facebook and others, including Twitter and Google-owned YouTube, to embrace a truth-squadding role they have long avoided ― in no small part out of concern that political figures might see their decisions as biased.

Facebook’s decision to overlook deception in political advertising came to light in a speech last month by Nick Clegg, the former British deputy prime minister who’s now Facebook’s vice president of global affairs and communications. He compared the social media platform with its more than 2 billion global users — a total not including the company’s subsidiaries Instagram and WhatsApp — to a tennis court.

“Our job is to make sure the court is ready — the surface is flat, the lines painted, the net at the correct height,” Clegg said. “But we don’t pick up a racket and start playing. How the players play the game is up to them, not us.”

Clegg did acknowledge a role for Facebook to combat outside interference and to make it clear to the public who is buying political ads, but did not mention a place for umpires to determine what is demonstrably false or otherwise out of bounds.

“It is not our role to intervene when politicians speak,” Clegg said. “That’s why I want to be really clear today – we do not submit speech by politicians to our independent fact-checkers, and we generally allow it on the platform even when it would otherwise breach our normal content rules.”

Facebook, which declined to comment on how it formulated the policy, confirmed it formally put it in place in September 2018, before the congressional midterm election. But even before that, some of the company’s fact-checking partners said they had never been asked to review the veracity of political ads that appeared on its platform.

The potential for abuse became clear Monday after Biden pressed Facebook to remove an ad based on demonstrable falsehoods. The ad claimed that Biden had used the threat of withholding $1 billion to Ukraine to quash an investigation of a company for which his son was a board member — even though the claim has been repeatedly labeled as untrue by news organizations. In response, Facebook said that “direct speech” by politicians was not covered by the fact-checking process that Facebook has put in place in recent years.

“Our approach is grounded in Facebook’s fundamental belief in free expression, respect for the democratic process, and the belief that, in mature democracies with a free press, political speech is already arguably the most scrutinized speech there is,” Katie Harbath, the public policy director for Global Elections at Facebook and a former digital strategist for Republican political committees and the 2008 presidential campaign of Trump’s personal lawyer Rudolph W. Giuliani, wrote to Biden campaign officials. “Thus, when a politician speaks or makes an ad, we do not send it to third party fact-checkers.”

That means posts and campaign ads by politicians operate in an alternative system. Ordinary users who push lies online, especially in a coordinated manner, now face a range of consequences, up to an outright ban from the platform. Politicians, however, largely do not. In doing so, Facebook found itself in the company of other social media sites, including Twitter, where the Trump campaign also ran its anti-Biden ad.

Twitter confirmed the ad did not violate its rules. YouTube spokeswoman Alex Krasov said political advertisers must meet certain verification and disclosure guidelines, but pointed to the site’s rules, which do not prohibit outright falsehoods in political ads.

Trump’s campaign spokesman Tim Murtaugh defended the ads as “100 percent accurate” and said, “The truth hurts and it’s not a surprise that Biden doesn’t want anyone to hear it.”

The claim is false: Biden’s move did not imperil the investigation, was backed by the State Department and was coordinated with leaders of the European Union and the International Monetary Fund due to concerns the prosecutor general ignored corruption, fact-checkers have stated.

Facebook’s exception is rooted in the philosophical tradition of the First Amendment and the typically wide berth the U.S. legal system affords political speech, but the practical consequences have enraged critics. Chris Hughes, the Facebook co-founder who has become a fierce critic of the company, tweeted Tuesday that Zuckerberg’s decision “to allow outright lies in political ads to travel on Facebook” was tantamount to “embracing the philosophy behind Trumpism and thereby tipping the scales.”

The company’s exception for political speech also has fueled resentment among the Democratic campaigns fighting to win attention and combat falsehoods online. Calling on Facebook to “refuse to air baseless ads,” Sabrina Singh, a spokesperson for Sen. Cory Booker (D-N.J.)’s presidential campaign, said in a statement: “We should have all learned the lessons from 2016 and social media companies should have to bear more responsibility to prevent lies from being spread against anyone.”

Inside the Democratic National Committee, officials have agonized over Facebook’s resistance to policing falsehoods, with some saying they feared the decision could effectively declare open season for political disinformation.

The kinds of falsehoods that they argued helped deliver Trump the presidency are being used more tactically now to achieve maximum attention and audience. Some officials said they are concerned about a rapid expansion in false ads now that campaigns realize they’ll face no consequences for bending the truth.

DNC chief Seema Nanda said in a statement that Facebook had missed an opportunity to ensure its platform was a place voters could rely on to learn the facts. “Trump has an utter disregard for the truth, and allowing his lies to go unchecked sets a dangerous precedent that could lead to further escalation,” she said.

Trump’s 2020 campaign has invested heavily in social-media advertising, including a system to test what Facebook ads might resonate with voters most. Roughly two-thirds of the $1.1 million spent on Facebook ads last week related to impeachment came from Trump’s campaign, according to data from the research firm Bully Pulpit Interactive.

Over the last two weeks, the Trump campaign has spent huge sums on Facebook ads, the data show, including $45,000 related to Biden; $252,000 related to socialism; $104,000 related to Mueller; and $86,000 related to “fake news.” Trump’s campaign spent more on Facebook ads last week related to the debates than any of the Democrats actually participating in them. And on the topic of guns and immigration, his campaign spent more on Facebook ads last week than all of the Democratic candidates combined.

“There will be a wild West in terms of what Trump is allowed to get away with,” said Brian Fallon, the former press secretary for Hillary Clinton’s campaign and now leader of advocacy group Demand Justice.