Twitter flags posts by Trump that made premature claims of victory or baseless ones about election fraud.

For the sixth time in less than 24 hours, Twitter on Wednesday flagged tweets by President Trump for violating its rules because they included unsupported claims of widespread election fraud and premature declarations of victory in key battleground states.

Twitter attached warnings to each of the president’s tweets, but also to others posted by his allies, including, among others, his official campaign account, the White House press secretary Kayleigh McEnany and the president’s son Eric Trump. All three had, like the president, claimed victory for Mr. Trump in Pennsylvania, a race that no major news organization has said is decided.

At least three of Mr. Trump’s posts were hidden and one was partially hidden, but each allowed Twitter users the option of viewing them. The platform also restricted the ability of users to retweet or repost the posts by the president that had violated the company’s standards.

Twitter’s actions came as Mr. Trump’s possible path to re-election narrowed, with former Vice President Joseph R. Biden Jr. winning Wisconsin and Michigan, two states in the so-called Midwest blue wall that Mr. Trump had won in 2016.

Mr. Trump falsely claimed on Wednesday afternoon that he had won not only Pennsylvania but also Georgia and North Carolina, states in which he had been leading in early vote totals but where a significant number of ballots remained uncounted. In each case, his lead was shrinking.