Top advisers on Sen. Bernie Sanders Bernie SandersThe Hill's Campaign Report: Trump faces backlash after not committing to peaceful transition of power Bernie Sanders: 'This is an election between Donald Trump and democracy' The Hill's 12:30 Report: Trump stokes fears over November election outcome MORE's (I-Vt.) presidential campaign are pushing back against what they say is a misleading narrative emerging around their candidate's prospects, arguing that biased media coverage and flawed polling are to blame.

In a phone call with reporters Monday, three senior campaign officials vented frustration with coverage of the senator’s campaign and argued that, contrary to recent headlines, Sanders has been gaining ground in the White House race, bolstered by a strong performance in last month’s Democratic primary debates in Detroit.

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“We’re sort of in the phase called the ‘Bernie write-off,’” said senior adviser Jeff Weaver. “There seems to be a direct correlation between the media coverage of the polls and Bernie Sanders’s standing in those polls.”

“Polls are one thing, but the energy that is on the ground is most important,” Nina Turner, one of the co-chairs of Sanders’s campaign, said.

The remarks came days after a Monmouth University poll of likely Democratic caucusgoers in Iowa showed Sanders slipping into fourth place in the Hawkeye State. That same survey found Sen. Elizabeth Warren Elizabeth WarrenHillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns On The Money: Anxious Democrats push for vote on COVID-19 aid | Pelosi, Mnuchin ready to restart talks | Weekly jobless claims increase | Senate treads close to shutdown deadline Democratic senators ask inspector general to investigate IRS use of location tracking service MORE (D-Mass.) rising to second place behind former Vice President Joe Biden Joe BidenPelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Hillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns Fox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio MORE.

Sanders’s aides and allies have long argued that he has been underestimated in the Democratic primary field, pointing to his unexpected near-win in the Iowa caucuses in 2016.

Media bias and skewed polling, they argue, have fueled inaccurate perceptions of the campaign and have failed to take into consideration the senator’s massive base of donors, volunteers and supporters who are not always reflected in public polls.

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Ben Tulchin, a pollster for the campaign, singled out the Monmouth poll on Monday, arguing it gave too much weight to older, establishment-minded Democratic voters in Iowa, without accurately taking into account the coalition of young, more diverse voters who make up Sanders’s biggest bloc of support.

“It was too old, it was almost two-thirds over 50 [years old],” Tulchin said. “It tilted older Democrat and more Democratic establishment-type voters.”

“Bernie does better with younger voters and voters outside the normal party system,” he added.

Tulchin pointed to a recent analysis of post-debate polling data by the website FiveThirtyEight showing Sanders gained more ground than any other candidate — about 1.8 percentage points — in the days after the Detroit debate. That same analysis showed support for Biden and Sen. Kamala Harris Kamala HarrisHundreds of lawyers from nation's oldest African American sorority join effort to fight voter suppression Biden picks up endorsement from progressive climate group 350 Action 3 reasons why Biden is misreading the politics of court packing MORE (D-Calif.) slipping post-debate.

“We started watching the polling data very carefully post-debate and it was a clear trend that Bernie was doing better,” Tulchin said. “No one was reporting that, so that was one of the impetuses of this call.”

Patrick Murray, the director of the Monmouth University Polling Institute, defended the recent Iowa poll, saying that even if it sampled more young voters, it would still show a downward trend for Sanders. He told The Hill that much of the backing that Sanders has lost in recent months has been “soft support” from voters who have not yet committed to backing a single candidate.

“Either way you argue it, the trajectory is he lost some of the soft support he had in April,” Murray said. “His core support remains with him, but it’s the people that move around that have drifted towards Warren. Could they move back towards Sanders? Absolutely.”

Polling in caucus states can be particularly difficult because turnout is often unpredictable and voters can — and do — change their mind during the caucus itself.

Murray said that many Sanders supporters do not have a history of participating in caucuses, meaning they are even more difficult to survey.

“Sanders's support is very difficult to poll, because it’s generally a younger activist base that doesn’t normally participate in these caucuses,” he said. “We found that in 2016, and we had to make some adjustments to keep up with that.”

There are reasons for Sanders to remain optimistic about his prospects, his advisers said. Health care, a topic where Sanders has asserted himself as a leader, is among the top concerns for voters. And Sanders has established himself as a political force in the Democratic Party, setting the tone on a number of issues, including "Medicare for All" and workers’ rights.

Weaver pointed to Sanders’s massive political operation in Iowa to argue that polling has not reflected his chances of success come caucus day in early February. He said the campaign has an army of volunteers on the ground and has already begun a direct mail program in the state.

Still, other candidates have gained on Sanders in recent months, most notably Warren, whose rise in the polls has closely correlated with Sanders’s decline. Harris has also seen a bump in recent weeks and is ramping up her efforts in Iowa. Biden, meanwhile, remains the candidate to beat, with most polls showing him as the front-runner.

Part of Sanders’s challenge may owe to the fact that he is no longer the sole standard-bearer of the progressive wing of the Democratic Party, one Democratic consultant said. Other candidates, like Warren, have seized on his message by offering proposals for single-payer health care and a $15-an-hour minimum wage.

“In some ways he’s the victim of his own success,” the consultant said. “It’s not novel anymore. Everyone’s putting new ideas on the table.”

Sanders has faced some media bias, the consultant said, but added that his campaign has to acknowledge the ups and downs of the nominating contest, especially in early primary and caucus states.

“It’s hard for them to argue otherwise,” the consultant said. “But I don’t think that means that Bernie’s out of the race. There’s just a great deal of fluidity in the Iowa and New Hampshire electorates. There always is.”

Weaver said that Sanders’s strength ultimately comes down to his grass-roots efforts, citing an anecdote from the Iowa Democratic Party’s Wing Ding Dinner from this past weekend.

“Every other campaign had their volunteers out front waving signs for the media,” Weaver said. “And where were the Sanders people? They were actually out there knocking doors in that community.”