https://apnews.com/e2560e949bd0d7413bd6244c6d4d51ca > OAKLAND, Calif. (AP) — Twitter has taken the unprecedented step of adding fact-check warnings to two of President Donald Trump’s tweets that falsely called mail-in ballots “substantially fraudulent” and predicted a “Rigged Election.” On Wednesday, the president threatened to impose new regulation on social media companies or even to “close them down." > But Twitter’s move and Trump’s reaction raise a host of questions, including why Twitter acted now, how it decides when to use such warnings and what its newly assumed role means for the 2020 U.S. presidential election.