Twitter Inc. has applied new rules against misinformation to a video circulated by one of President Trump’s top aides showing Joe Biden at a campaign rally saying, “We can only re-elect Donald Trump.”

Mr. Biden used those words but the video, which was retweeted Saturday by Mr. Trump, omitted what the leading Democratic presidential contender said in full.

“We can only re-elect Donald Trump if in fact we get engaged in this circular firing squad here,” Mr. Biden said during a speech Saturday in Kansas City, Mo., that called for party unity to defeat Mr. Trump. “It’s got to be a positive campaign, so join us.”

On Sunday, Twitter applied the label “manipulated media” to the bottom of the video, part of a new policy to combat misleading altered media on its platform.

Twitter’s new rules state that users “may not deceptively share synthetic or manipulated media that are likely to cause harm.” The company may also “label tweets containing synthetic and manipulated media to help people understand the media’s authenticity and to provide additional context.”