Inventing 21st-century presidential politics has been a seesaw process so far, with the energy and innovation coming from the party out of power. If that pattern holds, what whipsaw of change might be in store for 2020?

To review: In 2000 and 2004, Republicans broke with traditional wisdom that national elections are won from the center. While their candidate, George W. Bush, certainly gestured to centrists by presenting himself as “a uniter” and a “compassionate conservative,” political guru Karl Rove led a different strategy down in the trenches. By creatively crunching troves of newly accessible digital data, Rove’s team unearthed Republican-inclined voters who had never thought of themselves as partisans. By targeting them with messages and invitations, Rove tugged them into the GOP base.

It was basic 20th-century politics — identifying supporters and turning out their votes — but with an overlay of digital-age “big data.”

Barack Obama took that a further step in 2008. He perceived that the new tools of communication favor personal connections over institutional ties. Rather than use technology as Bush had done to identify potential party supporters, he used it to find Obama voters and drive them to the polls. An inexperienced backbench senator thus unhorsed the party front-runner, Hillary Clinton.

This was a personal victory, not a party win. Obama remained relatively popular through eight years as president, but his Democratic Party sank into a slough of weakness at nearly every level of government, revived only by the arrival of Donald Trump.

Trump got there in 2016 by delivering the next tweak. His days in “reality” television taught him the power of melodrama to command attention, and smartphones allowed him to script the drama himself. Obama’s campaign, while personal, embraced a tradition of American progress. Trump, Mr. “I Alone Can Fix It,” proposed no context beyond himself. He didn’t defeat the Bush-Rove apparatus so much as he rendered it irrelevant.

These back-to-back personal victories by Obama and Trump have given us a world in which the path to power appears to be a DIY project. That’s why dozens of Democrats are either already in the race or said to be flirting with running for president this year. Digital tools have knocked down the barriers.

But could 2020 be the year when ideas strike back?

That’s the question that lies behind the rapidly growing movement for a “Green New Deal” among Democrats. Proponents of the idea — including the digitally astute freshman Rep. Alexandria Ocasio-Cortez, D-N.Y., and Vogue magazine’s trippy online explainer, actress Bria Vinaite — acknowledge this is a concept in search of substance. But fleshed out, the concept has the potential to disrupt presidential politics all over again by putting digital power in the hands of the grassroots.

In fact, at this point in the election cycle, the Green New Deal looks more like a digital insurgent — more like early Obama or Trump — than any human Democrat. It’s popping up with escalating frequency on Twitter and Facebook and energizing the party faithful.

It is vague and sweeping at the same time — like “Yes, We Can” or “Make America Great Again.” As its proponents explain, the Green New Deal begins with the fierce urgency of a world deeply threatened by climate change and demands responses on a scale worthy of the original New Deal — a scale that involves not only trillions of dollars in infrastructure and technology investments but also a fundamental reordering of economic and political power in America. It includes more windmills and fewer cars, sure. But it could end up including vastly higher top tax rates and guaranteed jobs for all. It will not accept business as usual.

With polls showing that Democrats are more favorably inclined to socialism than capitalism, these tech-savvy voters are going to organize around those beliefs. And there’s no telling how far left they might go.

An ideological, rather than personal, insurgency is unfolding. Political reporters will be smart to watch it at least as closely as they watch the human traffic jams on debate stages from Iowa to South Carolina to California. Given how quickly things happen on the internet, the nascent movement already has the potential to define the 2020 race. With a following as deep and fervent as any individual candidate commands, the idea of a Green New Deal may compel would-be presidents to embrace the movement even before it is fully defined.

In other words, the great fight now beginning among Democrats might not be over candidates at all. The next innovation in digital politics may be the rise of a charismatic, untested agenda and the social media free-for-all to decide which concrete policies the agenda will contain. The Democrat who emerges from a very crowded field to carry the banner into next year’s election may well be the one who figures out how to throw a saddle on that rough beast and ride it.