But nearly four million digital signatures later, the philanthropist and environmental activist’s unlikely campaign has seized on an issue — impeaching Trump — that could become part of the Democratic mainstream in 2018. It’s placed at his fingertips a potentially powerful tool: an email list of millions of motivated activists who he can reach instantly for organizing and fundraising and that could become the hottest trove of data in Democratic politics since the email list that Bernie Sanders’ insurgent campaign against Hillary Clinton collected in 2016.

Steyer’s digital success is fueling intense curiosity about what he’ll do with that tool in the future — and whether he’ll use it beyond his California base, for a White House bid of his own.

“That’s how you build a grassroots operations for a presidential campaign. And if you are that guy who started this, that’s certainly a leg up in organizing a 50-state strategy,” said Mike Madrid, a Republican consultant in California who is advising former Los Angeles Mayor Antonio Villaraigosa, a Democrat, in his gubernatorial campaign. “That’s what he’s building, and it’s probably second only to Bernie Sanders’ list, and may be eclipsing it. Every election cycle has its own dynamics, and whoever is tapping into the sentiments of their own base usually has an advantage.”