Washington (AFP) – President Donald Trump’s threat to shut down social media companies after Twitter labeled two of his tweets misleading sets up a fresh challenge for platforms as they struggle to deal with political misinformation during a toxic election campaign.Twitter on Tuesday targeted tweets in which the president said that mail-in voting would lead to fraud and a “rigged election” in November, the first time the platform has placed a warning label on Trump’s comments.The president’s angry response and threat to “strongly regulate” or “close down” social media firms highlights the conu…