As he opened a presidential campaign office here Saturday, Sen. Bernie Sanders drew attention to the silence on President Obama’s trade push from the candidate who is far outpacing him in the polls: Hillary Rodham Clinton.

“I don’t understand how you don’t have a position on this issue,” he said.

On Sunday, Clinton had something to say — while still leaving herself plenty of wiggle room. She told a crowd at the first Iowa rally of her campaign that Obama should address the concerns of House Minority Leader Nancy Pelosi (D-Calif.) and the Democratic rank and file in Congress, who want the Trans-Pacific Partnership trade agreement to include better protections for American workers. Without improvements, she said, “there should be no deal.”

Clinton also drew a particularly sharp contrast with Obama — and, for that matter, everyone else who’s running to succeed him.

“No president would be a tougher negotiator on behalf of American workers, either with our trading partners or Republicans on Capitol Hill, than I would be,” Clinton said. However, she did not explicitly say whether she supports fast-track legislation known as trade promotion authority.

Late Sunday, Sanders slammed Clinton’s indecisiveness and said she should take a clear stance on fast-track authority. “I am not clear, nor do I believe the American people are clear, as to what Secretary Clinton’s position is. Is she for it, or is she against it? Those are your two options,” he told reporters in Indianola, Iowa. Not choosing, he said, “is not leadership.”

The exchange illustrates the unique challenge, and perhaps opportunity, that Sanders poses for Clinton, who polls 50-odd points ahead of the iconoclastic senator from Vermont.

Across Iowa, Sanders has drawn enthusiastic, swelling crowds of liberal Democrats hungry to hear the self-described socialist rail against the “continuing greed” of the nation’s corporate elite and against the Asian free-trade pact Obama is trying to push through.

Martin O’Malley, the Democratic former Maryland governor waging a long-shot campaign, is trying to make himself part of the discussion, too. For weeks, he has been needling Clinton on trade, including during a swing through Iowa on Thursday.

Shortly after Clinton’s Sunday speech, O’Malley issued a statement saying, “It’s time for leaders of the Democratic Party to step up and urge Congress not to fast-track this bad trade deal.”

The Democratic primary race is beginning to come alive, at least here in Iowa, where the first caucuses are eight months away and activists are champing at the bit for a contest.

Sen. Bernie Sanders, left, and former secretary of state Hillary Clinton. (AFP/Getty Images)

Clinton’s campaign moved to a new phase this weekend, from the intimate roundtables and house parties that defined her first two months as a candidate to big, boisterous rallies like the one she staged here Sunday afternoon in front of a giant American flag at the Iowa State Fairgrounds.

Sunday’s event was orchestrated to display grass-roots energy behind a candidate who has struggled to shed her aura of inevitability. For more than an hour before Clinton arrived in her motorcade, a bluegrass band played while guests ate burgers and drank lemonade at long picnic tables. Staffers were omnipresent, signing up volunteers and persuading supporters to commit to caucus on her behalf. The whole affair had a bit of a corporate feel — the kind of rally that costs big bucks — though the roughly 700 attendees seemed genuinely enthused about Clinton’s candidacy.

“Oh, this energy is real,” said Sue Dvorsky, a former Iowa Democratic Party chairwoman. “Here’s how you know it’s real: the noise. I’m a classroom teacher. When you walk into a gym full of kids, you listen for the sound. This is a genuine buzz.”

Once she arrived, Clinton stirred her supporters with partisan slams against Republicans — on tax policies, climate change and gay rights. She drew rousing ovations, especially when she referenced the historic promise of her candidacy: “I may not be the youngest candidate in this race, but with your help, I will be the youngest woman president.”

Liz Reams, 20, a student at Iowa State University, said Clinton was refreshing. “So many of the candidates are just old, white men,” she said.

The Clinton campaign has been methodical in its efforts to organize likely caucusgoers here. There are 27 paid field staffers and nine offices, with more likely to come. On Saturday, volunteers hosted 55 house parties across the state to watch simulcast videos of her kick-off speech in New York City and her appearance at a house party in Sioux City, Iowa.

State Rep. Marti Anderson hosted one such gathering at her home in Beaverdale, a Des Moines neighborhood that was so active for Obama in 2008 that it took on the moniker “Obamadale.” With a few dozen of her neighbors gathered in her living room and kitchen, Anderson gave a moving testimonial to Clinton’s character.

“I think Hillary is among the best of us,” she said. “You will hear all kinds of things about Hillary that don’t sound like the best of us, but I don’t see her as having any mean-and-nasty, horrible, hating everybody or anybody in her system. I think she’s a person that truly loves every human being.”

By contrast, the Sanders road show is drawing big crowds with little apparent organizational effort. Some are passionate die-hards, others merely curious. Friday night, he got a rock-star-like reception from about 800 people at a town meeting here at Drake University.

“You know, sometimes our campaign has been referred to as a fringe campaign,” Sanders said. “Well, if this is fringe, I would hate to see mainstream.”

Roughly 250 folks showed up to see him at a union hall in Marshalltown on Saturday and 300 more at a steak fry that night in Cedar Rapids. And Sunday morning, more than 500 people streamed into a theater in sleepy downtown Waterloo to size up Sanders.

“Whoa, we’re going to have to knock down some walls there,” the candidate said as he took the stage.

Sanders, who lacks Clinton’s polish and choreography, gave a rambling talk and took questions for 90 minutes. He drew repeated bursts of applause and calls of “yes” from audience members as he made the case that the country’s economic elite has hijacked its politics.

“Continuing greed is destroying this country, and it has got to stop,” Sanders said. “What we cannot sustain is a nation where so few have so much and so many have so little.”

He rattled off specific items on his policy agenda: boosting the minimum wage, guaranteeing 10 days of vacation time for workers, launching a major federal jobs program, breaking up big banks, financing elections with public money, reforming immigration policies, moving to a single-payer “Medicare for all” health-care system, providing universal pre-kindergarten programs and free college education, and so on.

After driving 90 minutes to see Sanders in Waterloo, Terry Pensel left as a true believer. He carried several yard signs and bumper stickers out with him.

“I don’t want to bash Hillary Clinton or any of the other Democrats, but I don’t see them having a plan to deal with the concentrated wealth,” said Pensel, a 47-year-old Web developer. “They’re giving lip service to a lot of the issues Bernie has been talking about for a long, long time.”

Tara Monson, 31, who conducts audits for a goat farm, brought her 6-month-old daughter to see Sanders in Marshalltown. She had a homemade “Feel the Bern” T-shirt.

“I never really heard of Bernie Sanders until he started running,” she said, “and he’s really grabbed me.”