Donald J. Trump has exploited social media like no other American president, using it as a springboard to change policy, as a cudgel against critics and as an outlet for self-affirmation. “He needs to tweet like we need to eat,” said Kellyanne Conway, his White House counselor.

Along the way, he has lent credibility to unsavory Twitter accounts through his habit of retweeting posts that catch his attention, seemingly without regard for who is behind them or their motives.

In three articles, The New York Times analyzed Mr. Trump’s posts, studied the accounts he follows and interviewed dozens of administration officials, lawmakers, Twitter executives and ordinary Americans caught up in his tweets. Here are some of our findings.

Attack, attack, attack — with a notable exception

Over half of the president’s more than 11,000 tweets are attacks, aimed at everything and everyone from the Russia investigation and the Federal Reserve to black football players and Amazon’s founder, Jeff Bezos. But in more than 2,000 tweets, Mr. Trump has cited one person for praise: himself.