WASHINGTON — As Democrats rallied opposition to Senate Republicans' health bill last week, Cory Booker turned to his phone to whip up a storm.

The New Jersey senator, joined by Rep. John Lewis (D. Ga.), a civil rights icon, walked to the steps outside the U.S. Senate, and sat to speak about their objections while streaming the conversation live on Facebook.

Soon, liberal activists and policy experts started stopping by. Other Democratic senators dropped in, sitting on the steps, ties loosened, and talking to the camera as a crowd built – eventually growing into a few hundred fired-up liberals. What began in early evening sunlight ended more than three hours later under the yellow glow of the Capitol's lights.

By Friday morning, more than a million people had watched the video on Booker's Facebook feed.

The event reflected Booker's talent for social media, and a long-calculated strategy: By cultivating an audience with everyday interactions, he has amassed a following he can reach when the big issues arise.

"It's why I work so hard on social media," Booker, 48, told me last summer. "While I may put up quotes and things like that, I'm really trying to build audiences so I can have a conversation about things that matter."

This was almost exactly a year ago, as Booker barnstormed the country and auditioned to be Hillary Clinton's running mate. Sitting in a greasy South Florida burger joint, the vegan senator ate a lunch of hummus and vegetable slices and explained his social media savvy in the kind of language you might expect from a Silicon Valley entrepreneur.

At one point he referred to his "syndicated content" and pulled out his phone for an example: A Facebook Q-and-A that drew nearly 600,000 viewers. That's more than many cable news shows capture, he said.

Fast forward a year. With Democrats battling a GOP-controlled Congress and starved for national leadership, Booker has emerged as one of the party's biggest voices, and regularly appears on lists of potential 2020 presidential candidates. His social media skills are part of his national brand.

His message isn't usually much different from what other Democratic senators are saying, but his oratory and fame amplify those arguments in ways few other politicians can.

"He's one of the best messengers the party has right now," said Adam Jentleson, a former deputy chief of staff for Harry Reid, the recently retired Democratic Senate leader.

Just about every lawmaker, of course, uses Twitter, Facebook, and other social media. President Trump tweets nearly every day to work around journalists and hammer out messages that are quickly echoed by his supporters.

But most political figures, Jentleson said, don't have the kind of following to make social media so potent. Among local senators, Bob Casey, of Pennsylvania, has the second-most Twitter followers: just under 169,000.

Booker has 3.1 million in large part because he works at it, delivering more than press releases and speeches.

"He engages in the spirit of social media, which is just checking in with your community and letting you know what he's doing that day," Jentleson said. He recalled how after one routine meeting Booker pulled Reid aside for a Snapchat post. "Then, when you have something to say that is substantive, a message you want to convey, they're there to listen."

The other key, he and others said, is that Booker does it himself, not through aides. As with Trump, followers know they are getting an unusually unfiltered view of an official's thoughts.

"What he does, that is successful — he's authentic," said Sen. Brian Schatz (D., Hawaii), a friend who joined Booker on the Capitol steps. "When you see him tweeting about something, you know it's him."

Booker has long had a talent for grabbing the spotlight. He rose to national prominence when his 2002 run for mayor of Newark of was featured in a documentary, Street Fight. His exploits in Newark — rescuing a woman from a burning home, chasing after a robber — built an unusual mystique around the mayor of a long-struggling city. He has been on The Daily Show, The Colbert Report, Oprah, Cake Boss, and starred in a TV series about his tenure, Brick City.

Schatz had never met Booker before the Garden State lawmaker joined the Senate, but "just knew him from being a famous New Jersey mayor."

There is a danger on social media, Booker conceded, of building a bubble of followers who might leave an impression of support that doesn't reflect the broader electorate.

But it can also help create surprises, as it did on the Capitol steps.

Booker said this week he told his colleagues about his plans a few hours in advance on the Senate floor, and aides called activists who could join in. Many others, he said, just heard about it and dropped by. The head of MoveOn, a progressive political organizing group based online, was at the Capitol for something else, and joined in (accidentally sitting on Casey's jacket at one point).

Booker, holding his phone, worked the event like a talk show impresario, building up each guest, interacting with the crowd — he hailed Philadelphia's vegan dining scene when one woman said she was from the city — and cracking jokes.

"Extra points if you can tell Durbin what a Snapchat is!" Booker called in reference to Illinois Sen. Dick Durbin, 72.

It was only one element of Democrats' push against the Republican health plan. There were also speeches, press conferences, and parliamentary delays. And in the end the biggest hurdle for the GOP has been internal divisions, not Democrats.

But Booker and his allies hope to keep the pressure on. Two days after his Facebook event, he went the traditional route, speaking at a Capitol Hill rally.

As he left, he was swarmed by young people — they all wanted a selfie.