Among Democrats, another convention hums in Sanders fans' phones

There is the Democratic convention unfolding onstage in Philadelphia this week, broadcast to millions on television. Then there is a second convention that exists largely hidden from public view in the smartphones of Bernie Sanders' delegates.

Using various messaging apps and secret Facebook groups, many of the delegates pledged to the U.S. senator and runner-up in the party's presidential nomination contest have been coordinating among themselves in Philadelphia.

This is where they debate how to show their opposition to Hillary Clinton, who will formally accept her nomination on Thursday as the Democratic Party's candidate for the Nov. 8 election but who is viewed by many left-leaning Sanders' supporters as too moderate.

There are arguments, flashes of paranoia and conspiracy as well. The messages often fly around too quickly to track.

"It makes your head explode, all these different apps," said Jessica Chambers, a Sanders delegate from Wyoming. She added one group using the Slack messaging app had more than 600 delegates. She poked at her iPhone, which had overheated and blacked out.

The clearest sign of the networks' power came on Tuesday evening, when at least 100 Sanders delegates quietly walked out in protest at Clinton becoming the nominee, after word of the plan spread through smartphones about an hour in advance.

At the time, Chambers was sitting near her Clinton-supporting friend Shelby Read, who said she is amused by the secrecy of the Sanders crew but supportive of their dissent.

"I never know what they're doing," Read told her friend later. "I thought people were going out to get food."

Many of the smartphone debates revolve around the question: What Would Bernie Do? But even when Sanders himself has weighed in, sending a mass text and email to his delegates on Monday evening urging against booing and back-turning, it is not always seen as an authoritative answer.

"Ignore that message," one skeptical recipient wrote in a message. "Bernie did not write it. Express yourselves freely."

The dissent in Philadelphia was fueled by leaked emails last week that showed the Democratic Party leadership tried to undermine Sanders during the primary campaign. In the streets of Philadelphia, thousands of angry left-wing protesters, including Sanders supporters, have marched, with some burning flags.

As hails of boos grew from the convention floor on Monday evening, despite Sanders' call for decorum, one delegate sent a warning around: "Stop booing and 'misrepresenting' Bernie and the campaign," it read.

The replies bristled. "We don't tell each other what to do," ran one.

Secrecy is encouraged among the members of the smartphone chat groups, and so delegates who showed messages to Reuters did so on condition of anonymity. They are wary of the party or security officials thwarting their plans.

The closest thing to a leadership structure is Coalition of 57, a small group of delegates that came together in the weeks before the convention. Members say they are in contact with officials in the Sanders campaign, but remain independent.

Jeff Weaver, Sanders' campaign manager, said he was not in contact with Coalition of 57, but otherwise declined to discuss the delegates' networks.

Coalition of 57 delegates were among those who, with mixed results, sought to tamp down the boos this week, suggesting instead to withhold cheers and applause, or wordlessly hold up protest signs during certain speakers.

"Let the silence be deafening," Dani Pellett, a coalition member for the Texas delegation, said.

(Reporting by Jonathan Allen; Editing by Alistair Bell)