The old CNN may have shied away from conflict; the new CNN is leaning into it. Once the down-the-middle nerd of the cable news playground, CNN — under the guidance of Mr. Zucker, a former sports and morning show producer with a yen for flood-the-zone programming — is now an elbows-out player in national politics, vociferously pledging to hold a truth-averse White House to account.

It’s a quarrel fueled in part by the yearslong, up-and-down relationship between Mr. Trump and Mr. Zucker, two outspoken television addicts who once enjoyed a rapport. They have known each other since the early 2000s, when Mr. Zucker put Mr. Trump in prime time as host of the NBC show “The Apprentice,” and both share an obsession with ratings and a love of spectacle.

Once in frequent touch, the men have not spoken to each other since December — “not a good conversation,” by Mr. Zucker’s description — and the dispute has bled beyond questions of coverage into the realm of corporate intrigue. Time Warner, CNN’s parent company, is preparing for a takeover by AT&T that requires approval from Mr. Trump’s Justice Department.