A CNN host called President Trump a “man baby” and a piece of excrement. (He used a more profane term, and later apologized.) A guest on “Fox & Friends” proposed that some Muslims in the United Kingdom be placed in internment camps, prompting an on-air disavowal from the hosts.

A Breitbart News writer argued on Twitter that “there would be no deadly attacks in the U.K. if Muslims didn’t live there,” generating angry replies. And Mr. Trump himself, in his first public comment about the attack in London on Saturday night, disseminated unconfirmed information from an unofficial source: The Drudge Report.

In the 24 hours after the deadly van and knife attack, the cycle of partisan broadsides and ideological combat that seems to dominate the media universe these days kicked into high gear.

News organizations turned against each other, with anchors on Fox News calling on CNN to punish its host Reza Aslan for his profane remarks about Mr. Trump in a Twitter post.