(CNN) A fresh series of Facebook ads this week by Democratic presidential candidate Sen. Elizabeth Warren seeks to put the social media giant on the defensive — by telling a lie.

The ads, which began running widely on Thursday, start with a bold but obvious falsehood: That Facebook and its CEO Mark Zuckerberg have endorsed President Trump's reelection campaign.

"You're probably shocked," reads the ad, which has already reached tens of thousands of viewers nationwide. "And you might be thinking, 'how could this possibly be true?' Well, it's not."

The ad's own admission of a lie seeks to draw attention to a controversial Facebook policy Warren has spent days criticizing. Under the policy, Facebook exempts ads by politicians from third-party fact-checking — a loophole, Warren says, that allows Zuckerberg to continue taking "gobs of money" from Trump's campaign despite Trump's ads telling untruths about former Vice President Joe Biden and his son.

On Saturday, Warren explained her ad was meant "to see just how far" the policy goes.

"We intentionally made a Facebook ad with false claims and submitted it to Facebook's ad platform to see if it'd be approved," she tweeted. "It got approved quickly."

Facebook's willingness to run the ad, she said, shows how the company prioritizes profits over protecting democracy.

Warren's escalating criticism of Zuckerberg highlights the backlash Facebook has faced as it has defended its policy on political advertising. Sen. Amy Klobuchar , another Democratic candidate, tweeted on Thursday that online platforms like Facebook need a "truth standard for paid ads." And Biden's campaign has compared Trump's ads to Russian-backed disinformation.

"Whether it originates from the Kremlin or Trump Tower, these lies and conspiracy theories threaten to undermine the integrity of our elections in America," Biden spokesman TJ Ducklo said on Tuesday.

In a statement Friday responding to Warren's ad, Facebook spokesperson Andy Stone said the company believes political speech should be protected.

"If Senator Warren wants to say things she knows to be untrue, we believe Facebook should not be in the position of censoring that speech," Stone said.

Facebook's policy on truthfulness in political ads has been in place for over a year, according to Nick Clegg, Facebook's vice president of global affairs and communications. In a recent speech, Clegg said Facebook demotes third-party content shared by politicians on its platform if it has been previously debunked by fact-checkers and rejects its inclusion in advertisements. That could include links, photos or videos created by others. But it does not cover misleading ads or other content created under politicians' own names.

The Warren ad puts Facebook in a challenging position, said Dave Karpf, an associate professor of media and public affairs at George Washington University.

"Either Facebook doesn't touch the ad and the ad is therefore noteworthy, or they touch the ad and it's noteworthy," he said. "It's a smart tactical move."

Warren has become one of Facebook's key antagonists after first calling for it and other Silicon Valley giants — such as Amazon, Google and Apple — to be broken up. But her rift with Facebook deepened after leaked audio published by The Verge revealed Zuckerberg fretting about the potential consequences of a Warren presidency.

"If she gets elected president, then I would bet that we will have a legal challenge, and I would bet that we will win the legal challenge," Zuckerberg is heard saying at a companywide meeting. "And does that still suck for us? Yeah. I mean, I don't want to have a major lawsuit against our own government. ... But look, at the end of the day, if someone's going to try to threaten something that existential, you go to the mat and you fight."

Warren responded via Twitter, "What would really 'suck' is if we don't fix a corrupt system that lets giant companies like Facebook engage in illegal anticompetitive practices, stomp on consumer privacy rights, and repeatedly fumble their responsibility to protect our democracy.

And earlier this week, she said Facebook "already helped elect Donald Trump once. Now, they're deliberately allowing a candidate to intentionally lie to the American people."

Other analysts say Facebook has made the right decision not to involve itself in regulating political speech. Lee Goodman, a former Republican chairman of the Federal Elections Commission, told CNBC Thursday that he believed Facebook's policy is "fair" because it "doesn't want to be the arbiter and doesn't want to take sides in these political debates."

Shannon McGregor, an assistant professor of communication at the University of Utah who has studied the impact of tech platforms on political speech, agreed that Facebook's policy provides greater transparency about its role in political discourse.

But she added that Facebook's efforts to remain neutral — particularly now, with respect to Warren — could itself affect the course of the 2020 campaign.

"The limits of what our political discourse is these days is, of course, largely shaped by these platforms where a lot of these politics happen," she said.