On X, formerly known as Twitter, the Biden campaign said it had joined the platform “mostly because we thought it would be very funny.” The decision marks a shift from the campaign’s previously stated position that it would not join the Trump platform, as reported by Axios in May.

Mr. Biden, who won the 2020 presidential election by narrow margins in just a handful of battleground states, is in search of any edge he can get with voters who could be persuaded to vote for him.

Voters who consume conservative media have long been considered a rich target for Democratic candidates. During the 2020 campaign, Democrats were split on engaging with Fox News, which party officials at the time said had more persuadable voters among its viewers than any other cable network had.

Senator Elizabeth Warren of Massachusetts called Fox News a “hate for profit” operation and refused its invitations for a town hall, while Senator Bernie Sanders of Vermont, an independent, and Pete Buttigieg, then a former mayor of South Bend, Ind., accepted. Mr. Buttigieg, now Mr. Biden’s transportation secretary, is often dispatched to explain the administration’s positions on Fox News.