Donald Trump's presidential campaign has baffled political experts, winning state after state, mostly without a traditional ground game of volunteers wearing buttons, holding signs and knocking on doors. Instead, Trump's grass-roots movement lives online, rallying supporters via social media.

Among those keyboard warriors are thousands of college and high school students, some of them not yet old enough to vote.

From his bedroom adorned with autographed Trump memorabilia he ordered online, Josh Gremillion, a lanky, blue-eyed senior at Clements High School in Sugar Land, administers a small arm of Students for Trump, a national organization rooted in social media and run by students with an adviser in Trump Tower. Since coming on board in January, the 18-year-old has orchestrated chapter start-ups in Sugar Land, Richmond, The Woodlands, the University of St. Thomas, Texas A&M University and Houston Community College.

"I've always been a conservative but never paid much attention to it," said Gremillion, Southeast Texas-area president of Students for Trump. "But this is our future."

Students for Trump sprouted on the same platform where Trump evolved his political stardom- Twitter - with one student and one account. Less than six months later, leaders say the group has 40 state directors, 225 chapters and 5,178 registered members. And it's planning to launch an online store selling shirts and gear.

"It's a good example of how social media is playing more and more of a role in organizing the campaign," said Candice Nelson, director of the Campaign Management Institute at American University. "Particularly with a demographic that historically has not been all that active in campaigns."

Just click 'share'

Gone are the days when grass-roots political activism meant staking yard signs and knocking on doors, said Kay Green, a senior consultant at RKG Marketing. Now it means clicking "share," and filling digital feeds with coverage of your candidate. Although political operations have been drifting online for almost a decade, attention has focused on how the candidates and campaigns wield their accounts. When it comes to grass-roots digital activism, Green said, no one does it better than young people.

"They know what to say, how to say and when to say it," Green said. "If you miss out on that market, you're going to fail."

Students for Trump started last September in a College Republicans meeting at the GOP state headquarters in Raleigh, N.C., where a speaker panel of elected officials inspired Ryan Fournier to get politically active on social media.

Fournier, a freshman studying political science at Campbell University in Buies Creek, N.C., made the Twitter account @TrumpStudents, after finding similar accounts for Trump's rivals. A month later he had 13,000 followers, and he got a call from an intern on the campaign.

"They wanted me to push out a bigger student movement," he said.

Astonished, he began to assemble a leadership team, starting with his friends and fellow Trump fans, like Andrew Nixon, an 18-year-old freshman at Brunswick Community College, now national field director of Students for Trump, who hopes to be a political consultant and took on the task of recruiting state directors through social media.

In late November, Fournier got a call from Guido Lombardi, a real estate developer and political consultant with an apartment in Trump Tower in New York.

"I'm an old friend of Mr. Trump," Lombardi said. "Ryan may have started something ahead of my phone call. But the moment I called him, that's when it started."

'Seeing him in person'

Students for Trump is just one of 340 grass-roots pro-Trump social media outfits, including websites, Twitter accounts and Facebook pages like Citizens for Trump, Trump Train and Stump4Trump, that Lombardi advises as a volunteer, independent of the Trump campaign.

It was August, during a long meeting in Trump's Manhattan penthouse office, Lombardi said, when he offered to shepherd the flock of social media accounts that sprouted to cheer on Trump's presidential bid.

"I just advise them on what's appropriate," he said. "You know: you can say this, you cannot say this."

Students for Trump rallied students across the country to start branches in their home states, like John McGrath, 17, a clean-cut high school senior from a military family in Weatherford, and a Trump fan since he saw the candidate speak at a September rally in Dallas.

"Just seeing him in person and the excitement and enthusiasm encouraged me to get involved," said McGrath, who plans to join the Navy and then get an MBA.

When Students for Trump tweeted a link to state director applications in December, McGrath applied, sent a résumé, got a phone interview and became head of the Texas operation. Now, he leads a team of deputy directors, regional presidents and 19 Texas chapters, with two more scheduled to start up Wednesday. He runs the state social media accounts, but he is recruiting to fill that role.

"Social media is a weapon and a tool," he said. "Everything we do is on social media."

In early January, Gremillion, who hopes to inherit his parents' real estate business, stumbled upon an Instagram post by the candidate that tagged Students for Trump. He clicked the link, contacted McGrath and got on board.

With a storm of text messages, tweets and Facebook posts, he spread the word that Students for Trump had come to Greater Houston, and aspiring volunteers tweeted back. By mid-March, there were five local chapters recruiting in their respective schools.

The area leadership team met in person for the first time on Thursday at a member's house in Missouri City.

"Honestly, Donald Trump is one of the best businessmen in the world. I just kind of grew up loving Trump," said Alicia Starko, 24, who heads a chapter at the University of St. Thomas, where she studies marketing.

"Me, too," said Amayah Brown, 18, head of Students for Trump for the Richmond area and a high school senior aiming at medical school. "I used to watch 'The Apprentice' every week."

While many older Trump supporters cite anger at Washington or frustration with traditional politicians, the students frequently cited Trump's business acumen as a main reason for their support and called his success a prime example of the American Dream. They recited most of the campaign's main talking points: that America needs a business-minded "outsider" in the White House, ready to "make deals," that it's time to "get tough," that Texas needs "the wall," and that political correctness has chilled the national discourse.

"People are repulsed by his unfilteredness," said Nathan Shultz, 25, who heads Students for Trump at Houston Community College, where he studies business. "But that's exactly what I love about him."

Said Gremillion, "He won't put up with ISIS or anything like that. He'll just take them out."

Groups voice faith

While the local groups waged a marginally traditional ground game ahead of Texas' March 1 primary - block walking and phone banking - their activism happens almost entirely online.

Gremillion said he likes to post articles about polls that evidence Trump's lead and maps with delegate counts showing what Trump has already won. Other leaders said "hashtagging" was key to effective online activism, because it helped online posts reach an audience outside their followers.

Five months after starting a simple Twitter account, Fournier met with Trump before a South Carolina campaign event in February to explain his group's progress. About three weeks later, Fournier made his first television appearance, speaking on TBS for Students for Trump.

Now, the group is planning to add Students for Trump shirts and stickers to the Trump gear store on their website, probably within the month.

Fournier, Gremillion and other group leaders voiced faith that they would be brought onboard the Trump campaign if New York real estate mogul wins the Republican nomination.

The Trump campaign did not respond to requests for comment.