(CNN) Sen. Bernie Sanders is preparing to renew his focus on early-voting states Iowa and New Hampshire, as frustrations grow inside his presidential campaign over slumping poll numbers and a series of taxing clashes between lower level staff and national leadership.

The campaign's in-house production team recently began shooting its first television ads of the 2020 cycle in Iowa, likely quieting at least one source of dissent among some senior aides, who have been pushing for Sanders to get on the air.

But concerns over the trajectory of Vermont independent's second presidential bid as new evidence emerges that Sen. Elizabeth Warren is beginning to peel away at his base are becoming more pronounced, along with a realization that a failure to launch in Iowa and New Hampshire could spell the campaign's early doom.

Sanders began his 2020 run dedicated to hammering away at the argument that he, more than any other Democrat, was best positioned to deny President Donald Trump a second term. But Sanders is no longer running as the party's lone progressive populist, as he did in 2016. Warren's emergence in that space, and the sense among some Democrats that she is a better bet to successfully unite the party than Sanders or Biden has altered the shape of the race.

The new dynamic came into sharp relief last weekend, when a CNN/Des Moines Register/Mediacom survey showed Warren surging into the top tier in Iowa with former Vice President Joe Biden. Sanders, who was well off the pace in third, trailed her in the state by more than 10 points, 22% to 11%.

Perhaps more worrying to some Sanders supporters, the Massachusetts progressive also overtook her colleague from Vermont with what has traditionally been his most reliable bloc of support: voters under 35, among whom Warren now leads in Iowa, 27% to Sanders' 22%.

Asked by a reporter on Sunday about the poll, Sanders said the campaign's internal numbers painted a different picture. But even if the senator and his staff have reason to believe the Iowa race is closer than the most recent polling indicates, interviews with two senior aides to Sanders revealed growing concerns that the Hawkeye State and New Hampshire could be at risk of slipping away.

The twin sources of frustration and internal debate, both aides told CNN, was a schedule that too often takes Sanders away from the early voting states and a perception among staff, especially in Iowa, that the campaign has been wrong-headed in its decision to put off investing there in television ads.

"It can't be this national campaign anymore. He's got to go on TV in Iowa. He's got to park himself in Iowa. He's got to show that he's the underdog fighting back," one of the aides told CNN.

The aide, who said that staff in Iowa has been "screaming for Bernie to get on TV" there, suggested that Sanders had overestimated his strength in Iowa and New Hampshire.

"I think there was initially this sense of, which candidates always want to do, they want to expand," the aide said. "And so I think there was a sense in the campaign, probably a little bit wrongly, that he had an Iowa and New Hampshire base and was going to focus on South Carolina and Nevada and California and expanding. And I think what this is showing is he's got to just go back to basics."

In an interview, campaign manager Faiz Shakir brushed off criticism that Sanders has spent too much time building a national brand, along with the suggestion that it had cost him in those early states. He pointed to Sanders' recent travels in Oklahoma, North Carolina and Colorado as evidence that the campaign is prepared for the long haul.

"We have four fingers in the air," Shakir said, referencing the start of the fourth quarter of 2019. "We are ready to finish strong and, with that, comes an adjustment in the time and attention we pay to certain parts of the map and that includes Iowa and New Hampshire."

But Shakir conceded that the shrinking calendar meant the campaign's margin for error was diminishing too.

"The arguments that we have framed up for the public, they need to come through clearly in this next period of time," he said of Sanders' efforts in Iowa. "I think delivering that message is absolutely critical here in the fourth quarter. We have put ourselves in a position to do very well."

A second senior aide also indicated that a move to focus more on Iowa and New Hampshire was imminent. That doesn't mean Sanders plans to eschew future trips to join striking union workers outside those states, as he did this week, but that Iowa, where he kicked off his "Bernie Beats Trump" tour on Monday, is soon likely to serve as a de facto base of operations.

Shakir also told CNN that Sanders, after parting ways earlier this year with the firm that made his ads in 2016, has personally decided to use the campaign's in-house team to produce his first round of television spots. The group, a fixture on the trail with the Sanders, began shooting elements for its first TV ad this week. Biden and South Bend, Indiana, Mayor Pete Buttigieg have already begun to run ads in the state. Warren on Tuesday announced a planned eight-figure TV and digital ad buy in Iowa, New Hampshire, Nevada and South Carolina.

"We have a major advantage over other campaigns in that we have a lot more cash on hand," Shakir said of the campaign's approach to ad expenditures. "You can use it correctly, or you can squander it. We are trying to stick to our strategy and make sure we are using our resources wisely."

The back-and-forth over the campaign's media strategy has been pointed, but both of the senior aides who spoke to CNN touched on a more fundamental source of internal tension: the increasingly public disconnect between state level operations, most notably seen in New Hampshire, and the campaign's national leadership.

Those conflicts have been stoked, the aides said, by mostly lower level staffers, many of them Sanders loyalists from 2016 who have bristled at the 2020 leadership's tone and more exacting standards.

A progressive consultant familiar with the campaign's operation in New England told CNN that unsettled Sanders diehards pushed, successfully, for the ouster of former New Hampshire state director Joe Caiazzo, who was reassigned to Massachusetts. Caiazzo, who declined to comment, has been replaced by Shannon Jackson, a more familiar face who managed Sanders' 2018 Senate race.

According to a new poll from Monmouth released Tuesday, the campaign is facing increased headwinds in New Hampshire, where Warren -- as she did in Iowa -- has surged into the top-tier with Biden. Sanders, again, sits third with 12% to Warren's 27%.

The progressive consultant also said the campaign's internal politics were complicating efforts to broaden its appeal and forge alliances with outside grassroots groups.

"It's very confusing," the progressive consultant said of communications with the campaign. "I have talked to people that hear from all of them -- that hear from (senior adviser) Chuck Rocha, that hear from (senior adviser) Jeff Weaver and that hear from Faiz. But my understanding is that if you want to get to Bernie, it's not Faiz that you go through. It's Weaver or (deputy campaign manager) Ari Rabin-Havt."

For new groups only beginning to make sense of the progressive firmament, the process of sorting out the campaign hierarchy can be a turn-off -- especially when Warren's team has developed a reputation for proactively seeking out those less seasoned activists.

One of the senior Sanders aides conceded that the growing pains have been, at times, sharper than many expected. In 2016, the aide said, the absence at times of a coherent national operation meant that field staff had much more autonomy. The shakeup in New Hampshire, and lower level changes in Iowa, were largely instigated by internal arguments rooted in differing ideas about how to run a successful campaign without betraying the spirit of Sanders' first run.

Shakir conceded that there is a level of angst between staff at the local level and the national office. But he insisted that the decisions being made are part of a desire to see the vision of the candidate executed.

In the end, he argued, the campaign team -- from top to bottom -- needs to trust in Sanders' judgment.

"We all take our leadership and follow the direction of Bernie Sanders and he drives the strategy, and the argument of how we are going to win people over," Shakir said. "I think to the extent that there's other people on campaign who have different views, I respect and appreciate that.

"But ultimately I believe strongly that we all take our guidance from the top -- and he is the one driving this train."