The gruff, disheveled and unabashedly liberal Sen. Bernard Sanders has emerged as the top challenger to front-runner Hillary Rodham Clinton, gaining momentum in early-voting states as more Democratic voters flock to his call for a political revolution from the left.
Mrs. Clinton still enjoys big leads in most of the polls, but the surge by Mr. Sanders in Iowa and New Hampshire polls last week separated him from the other long shot contenders for the Democratic presidential nomination and reminded the Clinton campaign to keep an eye on the rearview mirror.
Riding a wave of newfound popularity, Mr. Sanders barnstormed across New Hampshire this past weekend with seven events in two days.
The sudden rise in the polls not only confounded naysaying pundits, who had dismissed his campaign as a sideshow, but also took Mr. Sanders by surprise.
“It’s happening faster than I thought,” Mr. Sanders told WMUR News 9 during a campaign stop in Nashua, New Hampshire.
Democratic Party officials and liberal activists credited the rise of Mr. Sanders, a Vermont independent who proudly labels himself a socialist, to his forceful appeal for a grass-roots movement to fight Wall Street, income inequality, college debt and climate change.
“I think he’s going to be a big threat to Hillary,” said James Berge, Iowa Democratic Party chairman for Worth County. “For the middle class and working class, everything that Bernie stands for hits right at home for us.”
Some campaign strategists cautioned Mr. Sanders about the threat of peaking too early ahead of the Iowa caucus and New Hampshire primary, thus suffering a late-in-the-game ebb of enthusiasm that would allow Mrs. Clinton to maintain her lead or another rival to jump to the front of the pack and win.
Sanders campaign officials brushed aside the warning, saying they would remain focused on building upon the current momentum.
The Clinton campaign has insisted that they have always been prepared for a primary race, despite the widespread perception that Mrs. Clinton is the party’s all-but-inevitable nominee.
In Iowa, Mr. Sanders climbed to 26 percent in a Bloomberg Politics Poll released last week, up from previous polls showing his support in the mid-teens. He had nearly cut in half Mrs. Clinton’s lead.
The former first lady, senator and secretary of state captured 50 percent in the new poll, topping Mr. Sanders by 26 points. Her advantage over Mr. Sanders had shrunk from 41 points, when she had 57 percent support a month ago in a similar Bloomberg/Des Moines Register poll.
In New Hampshire, two polls showed Mr. Sanders on the rise.
He trailed Mrs. Clinton by just 8 points, 43 percent to 35 percent, in a WMUR/CNN Granite State Poll — his most stunning finish to date.
He climbed to 24 percent in a Bloomberg Politics poll of likely voters in New Hampshire’s first-in-the-nation Democratic primary. Mrs. Clinton won 56 percent for a 32-point lead in that poll, compared to a 44-point lead she held over Mr. Sanders in a similar poll in early May.
The other Democratic contenders — former Maryland Gov. Martin O’Malley, former Rhode Island Gov. Lincoln Chafee and former Virginia Sen. Jim Webb, were relegated to just 1 percent, 2 percent or less in the new Iowa and New Hampshire polls.
“Sanders certainly seems to be consolidating the non-Hillary vote,” said Craig Varoga, a Democratic campaign strategist who previously worked on the 1996 re-election campaign of Mrs. Clinton’s husband, former President Bill Clinton.
“Sanders has a Yoda thing going on, and his dishevelment makes him appear authentic, whether you agree or disagree with what he actually believes,” he said.
Neil Sroka, communications director for the liberal group Democracy for America, said Mr. Sanders’ support runs deeper than a protest vote against Mrs. Clinton.
“We are very early on in the process, but if anything, [the polls] show where the energy and momentum is in the Democratic Party,” he said. “It shows that that is where the base of the Democratic Party is right now, and candidates need to be speaking to that.”
Democracy for America was part of an effort to draft into the presidential race liberal firebrand and beloved champion of the left Sen. Elizabeth Warren of Massachusetts. But Ms. Warren refused to throw her hat in the ring.
Mr. Sroka said that the enthusiastic support for Ms. Warren among liberal activists would not simply transfer to Mr. Sanders because much of the appeal was for her personally. But he said Mr. Sanders had succeeded in igniting a similar movement within the party’s base.
“What Sen. Sanders and Sen. Warren share, and what attracts people to them, is that there is no doubt that what Bernie is running on is what Bernie believes in,” he said. “It’s more than authenticity — authenticity is part of it — but it’s also his clarity of vision and having a real consistent vision for how he will go about achieving the things he wants to achieve.”
“There is a similar enthusiasm behind Sen. Sanders. I think that’s where it comes from,” said Mr. Sroka.
Pete D’Alessandro, the Sanders campaign’s state organizer in Iowa, said that, beyond the polls, the swell of support for Mr. Sanders is evident in the large turnout at his events and the flood of volunteers.
The campaign signed up about 1,000 volunteers in the first month of organizing in Iowa, he said.
“Every crowd we’ve gotten has exceeded our original goals and exceeded our RSVPs and what we thought we would have the day of the event,” he said. “That tells you that there is something organic going on.”
Mrs. Clinton has mostly ignored her Democratic rivals, instead focusing her attacks on the field of Republican presidential hopefuls.
But Mr. Sanders’ rise did spur Clinton supporters to take aim at him.
Sen. Claire McCaskill, a Missouri Democrat backing Mrs. Clinton, said on MSNBC that Mr. Sanders was “too liberal” to win the presidency.
She said the media had given Mr. Sanders a pass by not mentioning that he is a socialist, though most political coverage of Mr. Sanders has included reference to it.
She also discounted the large crowds showing up to hear Mr. Sanders’ stump speech, comparing him to former presidential candidates Rep. Ron Paul of Texas and political commentator Pat Buchanan, who both attracted fervent followings but never captured the presidential nomination.
“It’s not unusual for someone who has an extreme message to have a following,” said Mrs. McCaskill.
Mr. D’Alessandro said that the Sanders campaign isn’t worrying about Mrs. Clinton’s campaign.
“I’m not being glib when I say this: We really don’t think about their campaign because we can’t,” he said. “We’re in this thing to maximize as many caucusgoers as we can and push this movement beyond Iowa and to the rest of the country.”