Trump has been grilling a Supreme Court decision that was made last week that will allow Pennsylvania to count votes until three days after the election, if they are postmarked with a date showing they were sent on November 3.
It could mean a delay in the result in Pennsylvania which is a key battleground state and could influence the election.
Tensions in Philadelphia, the state's capital, are already at large after the police killing of Walter Wallace Jr.
Trump on Monday night posted the same message on Facebook and Twitter, writing: "The Supreme Court decision on voting in Pennsylvania is a VERY dangerous one.
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"It will allow rampant and unchecked cheating and will undermine our entire systems of laws. It will also induce violence in the streets. Something must be done!"
Twitter replaced it with a warning which reads: "Some or all of the content shared in this Tweet is disputed and might be misleading about an election or other civic process."
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To be able to see the tweet, users must click on it - out of the president's feed - and open it. Users cannot like, comment beneath it, or share it.
On Facebook, the post was flagged with a counter-argument. "Both voting by mail and voting in person have a long history of trustworthiness in the US. Voter fraud is extremely rare across voting methods," their flag read, giving the Bipartisan Policy Center as the source.
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It is the latest in a series of occurrences whereby the two social media giants have played favorites with politics.
Last month, just two weeks before the polls opened, both sites censored a New York Post story that revealed provocative emails between Hunter Biden and a Ukrainian businessman that implied Joe Biden had met the businessman and had possibly even done him a favor when he was Vice President.
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It raised enormous questions of ethics and became a central point of the last presidential debate.
But Facebook, without giving any reason for it, claimed the story had to be checked by its own fact-checkers.
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They have never applied such skepticism to stories by notoriously left-wing outlets like CNN or The New York Times.
Twitter said the story violated its privacy policies. After a hassle from the right, they allowed it to be shared again.
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