In just the past week, 28,000 people showed up to see Bernie Sanders in Portland, Oregon, including about 9,000 people who were content to watch the Vermont senator on giant screens in the overflow area. Another 28,000 people joined him in Los Angeles.

All told, Sanders has brought his wonky brand of populism to 100,000 people at rallies and other events since July, easily outpacing Hillary Clinton, spurring comparisons to President Barack Obama's grass-roots appeal and even earning a few superlatives from Donald Trump.

Hillary Clinton launched her presidential bid on April 12 through a video message on social media. The former first lady, senator and secretary of state is considered the front-runner among possible Democratic candidates."Everyday Americans need a champion, and I want to be that champion -- so you can do more than just get by -- you can get ahead. And stay ahead," she said in her announcement video. "Because when families are strong, America is strong. So I'm hitting the road to earn your vote, because it's your time. And I hope you'll join me on this journey."

Ohio Gov. John Kasich joined the Republican field July 21 as he formally announced his White House bid. "I am here to ask you for your prayers, for your support ... because I have decided to run for president of the United States," Kasich told his kickoff rally at the Ohio State University.

Ohio Gov. John Kasich joined the Republican field July 21 as he formally announced his White House bid. "I am here to ask you for your prayers, for your support ... because I have decided to run for president of the United States," Kasich told his kickoff rally at the Ohio State University.

Sen. Ted Cruz of Texas has made a name for himself in the Senate, solidifying his brand as a conservative firebrand willing to take on the GOP's establishment. He announced he was seeking the Republican presidential nomination in a speech on March 23."These are all of our stories," Cruz told the audience at Liberty University in Virginia. "These are who we are as Americans. And yet for so many Americans, the promise of America seems more and more distant."

Businessman Donald Trump announced June 16 at his Trump Tower in New York City that he is seeking the Republican presidential nomination. This ends more than two decades of flirting with the idea of running for the White House."So, ladies and gentlemen, I am officially running for president of the United States, and we are going to make our country great again," Trump told the crowd at his announcement.

"You know what? He's getting the biggest crowds," Trump, the Republican presidential front-runner, said this week in Michigan. "We're the ones getting the crowds."

The droves of people packing arenas from Maine to Washington state to hear from a cantankerous 73-year-old senator who identifies as a democratic socialist reflect Sanders' unique ability to tap into the energy -- and anxiety -- of the left this election season. With his speeches bemoaning the power of Wall Street and entrenched politics, Sanders is becoming the undisputed front man for the liberal movement that aims to upend the status quo.

'Upset the apple cart'

"He is now a messiah on the left," said Greg Guma, who wrote "The People's Republic: Vermont and the Sanders Revolution," and has known Sanders since the 1970s. "He will upset the apple cart."

His impact on the race is becoming more acute. A Franklin Pierce University/Boston Herald poll this week found that Sanders is overtaking Clinton in New Hampshire, winning the support of 44% of voters compared to 37% for the former secretary of state. The findings aren't entirely surprising considering Sanders represents neighboring Vermont in the Senate. Still, they could give Clinton's camp pause at a time when she is also grappling with a host of other negative headlines related to her email scandal and polling that indicates the public doesn't trust her.

A CNN/ORC Poll of Democratic voters in Iowa released on Wednesday found Clinton remains dominant there, though Sanders is showing some areas of strength. Sanders outpaces Clinton when Democratic caucusgoers are asked who is more honest and the gap is narrowing when it comes to who would better handle the economy, though Clinton still has the advantage on that issue.

Sanders' massive crowds and growing momentum stand in stark contrast to Clinton's more low-key events and her summer full of campaign stumbles and email controversies. While the self-described democratic socialist may once have been seen as a long-shot contender, he is now seen as a credible alternative to Clinton by some liberals.

"It seems like Bernie is representing the counterculture views," said Michael Glenn, who has never voted but showed up at a rally for Sanders in Portland. "I think he represents a lot of the people who are kind of looking beyond Democrat or Republican."

He is also drawing nontraditional voters, suggesting he could either expand the party or simply become a political curiosity like Howard Dean and flame out for lack of deep support. At his events, die-hard progressives waving handmade signs about climate change and Arctic drilling mingle with veterans and people wearing National Rifle Association T-shirts.

Glenn, his long beard matching his even longer brown hair, said he approached politics from a "no party perspective" and that when he looks at the 2016 candidates, Sanders is the most obvious candidate for someone like him.

Democratic fissures

Sanders' candidacy is highlighting fissures among Democrats.

He counts unions among his biggest campaign backers, underscoring his role as a class warrior battling for the little guy. But his massive crowds are predominantly white.

"The Bernie voter is the Bill Maher viewer who is a little on the cynical side," said University of Vermont political science professor Garrison Nelson, who has known Sanders for years. "This is a chance to put a thumb in the eye of the establishment."

His campaign has become the most visible vehicle for the Occupy Wall Street movement, a group of activists who openly challenged Democratic leaders over their kid-glove approach to Wall Street. And they are rabid on social media, going after detractors and amplifying Sanders' message. His debate-night tweet slamming Republicans as "out of touch" was retweeted 33,000 times, the most of any other candidate.

Supporters say they are drawn to his authenticity and plain talk.

"You trust that he's telling you the truth," said Claire Met, an Oregon native who came to the Portland event with three friends.

Sporting a Sanders T-shirt, as well as a half-dozen Sanders buttons, Met said, "You can trust that he's not going to change his mind once he gets into office. And if he does change his mind there's going to be a darn good reason for it."

Brian Foren, a father who brought his young son Liam to a Seattle event, echoed Met, but added a concern.

"I worry that he's confused for being loony by being so honest. It shocks Americans," he said.

Clinton's prospects

Sanders has largely failed to catch on among African-American and Latino voters. One event over the weekend was disrupted by protests from leaders of the Black Lives Matter movement.

Clinton's prospects are bolstered by a firewall of African-American voters in Southern and big swing states. According to a recent Gallup Poll, 80% of African-Americans view Clinton favorably, with just 23% viewing Sanders the same way. Only 33% of African-Americans are even familiar with the Vermont senator, a metric the Sanders campaign has tried to improve. After a group of Black Lives Matter protesters shut him down at a Seattle rally, Sanders released a racial justice platform and announced the hire of Symone Sanders, a black staffer who will serve as campaign press secretary.

She has introduced Sanders at several rallies, hugging him as he approaches the podium.

Later this month, he will travel to Charleston, South Carolina, an early test of how well he can play on what should be Clinton's turf.

The Clinton campaign has largely avoided criticizing Sanders and Sanders has returned the favor.

When he was asked recently about whether voters should question Clinton's trustworthiness, Sanders said he admired Clinton, pointed out their differences and also pivoted to identity politics.

"Hillary Clinton has been under all kinds of attack for many, many years. In fact, I can't think of many personalities who have been attacked for more reasons than Hillary Clinton," he said on CBS' "Face the Nation." "And, by the way, let me be frank. And I'm running against her. Some of it is sexist. I don't know that a man would be treated the same way that Hillary is."

For Sanders, converting a summer fling into something more lasting will require transforming his buzzy appeal and big crowds to electoral wins in big states. Allies of Clinton concede he will likely win a handful of states, as no Democratic nominee in modern history who wasn't a sitting vice president has managed a clean sweep.

While he is unlikely to go negative against Clinton or other Democrats, his brash populism will infuse the nomination fight, adding energy as well as unpredictability to a race that some had thought would be a coronation.

"He is more polite than Trump, but there is a similarity there, it's a New York thing and an alpha male thing," Guma said. "Bernie is a Democrat with guts."

Henderson reported from Washington while Merica reported from Portland.