That campaigns are now being fought largely online is hardly a revelation, yet only one political party seems to have gotten the message. While the Trump campaign has put its digital operation firmly at the center of the president’s re-election effort, Democrats are struggling to internalize the lessons of the 2016 race and adapt to a political landscape shaped by social media.

Mr. Trump’s first campaign took far better advantage of Facebook and other platforms that reward narrowly targeted — and, arguably, nastier — messages. And while the president is now embattled on multiple fronts and disfavored by a majority of Americans in most polls, he has one big advantage: His 2020 campaign, flush with cash, is poised to dominate online again, according to experts on both ends of the political spectrum, independent researchers and tech executives. The difference between the parties’ digital efforts, they said, runs far deeper than the distinction between an incumbent’s general-election operation and challengers’ primary campaigns.

The Trump team has spent the past three years building out its web operation. As a sign of its priorities, the 2016 digital director, Brad Parscale, is now leading the entire campaign. He is at the helm of what experts described as a sophisticated digital marketing effort, one that befits a relentlessly self-promoting candidate who honed his image, and broadcast it into national consciousness, on reality television.