MANCHESTER, N.H. — Bernie Sanders, just weeks after a heart attack took him off the presidential campaign trail, renewing questions about his age and health, roared back last week with a strong debate performance and the disclosure of a quarterly fundraising haul that vanquished all of his Democratic competitors.

But the 78-year old Vermont senator, whose powerful oratory and progressive message on income inequality lifted him to serious contention in the 2016 Democratic contest against Hillary Clinton, is less formidable this time, with polls in early states and beyond showing his status as a top-tier candidate at risk.

From the challenge posed by fellow progressive Elizabeth Warren to staff clashes and poor strategic communication, Sanders has struggled to compete in a larger field and a new political environment. His health scare added another major challenge.

Other than Tuesday's televised debate in Ohio, Sanders has been largely off the trail since his heart attack Oct. 1. He held his first major campaign event since his hospitalization on Saturday, when New York Rep. Alexandria Ocasio-Cortez joined Sanders at a New York City rally to endorse his candidacy.

"I am more than ready to take on the greed and corruption of the corporate elite and their apologists," Sanders told thousand of supporters in Queens. "I am more ready than ever to help create a government based on the principles of justice, economic justice, racial justice, social justice and environmental justice."

He added, "To put it bluntly, I am back."

Sanders continues to lap the 2020 Democratic field in fundraising. He raised $25.3 million in the fiscal quarter that ended Sept. 30, and reported more than $33 million cash on hand. That tops Warren and former Vice President Joe Biden and all but assures Sanders has the resources to remain in the race for months to come.

Sanders was also the only Democratic candidate to receive more donations than President Donald Trump in the third quarter — he had 1.4 million individual donations to Trump’s 1.05 million.

Yet despite all these advantages, Sanders faces a steep climb back into contention.

The heart attack renewed questions of whether he has the physical stamina to continue a rigorous campaign, let alone serve for four or eight years as president.

Sanders' polling numbers, which had begun to slip in recent months as Warren picked up steam, showed a steeper plunge after news of his heart attack was revealed. A national poll shows Warren and Biden in a tight race at the top with 30 percent and 27 percent, respectively, with Sanders a distant third, with 11 percent.

Allies say the Sanders campaign's biggest challenge has been the lingering disagreements on how to retool strategy from his 2016 campaign for 2020. Those clashes have been especially apparent in New Hampshire, the high stakes first-in-the-nation primary state where he won in 2016 with 60 percent of the vote, but where the latest state polling has him in third place, trailing Warren and Biden by double digits even before his health scare.

"He's the only one who has fallen like a rock. From the high polling, and the high expectations, he's been brought back down to earth," said Susan Casey, a Democratic strategist and veteran of several presidential campaigns in New Hampshire.

Sanders' New Hampshire state director, Joe Caiazzo, was reassigned from his role to head operations in Massachusetts instead. Around the same time, Sanders' senior adviser and 2016 New Hampshire steering committee member Kurt Ehrenberg cut ties with the campaign.

"The clear, clear feeling permeating the Bernie Sanders campaign in New Hampshire was a sense of gloom, doom and frustration," said a former aide familiar with a private steering committee meeting held the day before a staff shake-up was announced last month.

Some staffers have also departed the campaign in Iowa, the first-in-the-nation caucus state. A recent Des Moines Register poll found Sanders in third in the state, again behind Warren and Biden. A post-heart attack poll of 18 early states through Super Tuesday on March 3 showed similar results.

From the start, the Sanders campaign faced the challenge of managing expectations in this year's diverse, crowded field of fresh faces.

And instead of expanding his devoted base from 2016, Sanders has struggled to maintain it, as many of his 2016 supporters are less enthusiastic about his second round.

One prominent Democratic activist familiar with the campaign said that Warren, who has released an extensive list of specific policy proposals, has simply offered a more compelling alternative in a crowded field.

Others say Sanders needs to do a better job of telling his story and countering the narrative that he and Warren are essentially the same candidate.

Current and former Sanders aides acknowledge Warren has outpaced them, both in driving a message and in campaign organization in early states. But they also point to policy proposals they say go much further than Warren's, like a more robust Green New Deal to fight climate change and a more aggressive wealth tax.

Aides also point to differences between the two candidates that can work to Sanders' advantage down the road, including a more diverse supporter base.

An NBC News/Wall Street Journal poll from September found Warren's support is strongest among liberal, college-educated white voters — an important constituency but far from the majority needed to win the nomination.

Sanders is more popular than Warren among younger voters and black voters and voters with less education. But for now, polls show black and less educated voters still favor Biden over Sanders.

"He's being squeezed on both sides, one by Warren and one by Biden. And he's just not making a compelling case for why people should come back to him," a top Democratic strategist familiar with the campaign told NBC News.

Yet many on Sanders' team remain bullish on his chances.

At a recent field office opening in Dover, New Hampshire, campaign co-chair Ben Cohen of Ben and Jerry’s Ice Cream said the reaction inside the campaign to Sanders’ heart attack has been one of revitalization, not worry.

"It was kind of like a rallying call. People are more psyched, more motivated than ever," he told NBC News. "Personally, I had quadruple bypass open-heart surgery. That's a big thing. This guy had a couple of little stents put in. That is not a big thing."

In South Carolina, the first primary state where African Americans play a pivotal role, aides say Sanders is in a much stronger position than he was in 2016 when Clinton's support among black voters helped her prevail.

"We are far ahead of where we were four years ago, in terms of staffing, in terms of activity," state communications director Michael Wukela said, adding that 72 percent of Sanders’ staff in the state are people of color.

Still, many voters who supported Sanders in 2016 say they’ve moved on.

Iowa City resident Dawn Harbor caucused for Sanders in Iowa in 2016 but said she is leaning heavily toward Warren this time.

"He is a lot more angry. She's a lot more level-headed," Harbor said. "I mean, Bernie's been doing the legwork forever, and we wouldn’t be here without him, that’s true. But in this moment, we need her."

And for other voters, Sanders' health is now front of mind.

Ania Blanchard, 19, told NBC News in the senator’s hometown of Burlington, Vermont, that she is still undecided but that Sanders’ heart attack gave her pause.

"It kind of makes me not want to vote for him quite as much just because I want someone in office who’s going to not be having to focus on their health as a main priority," Blanchard said.

Still, Sanders’ nationally televised debate performance last week was strong and may have calmed the concerns of some supporters. After the debate, senior campaign adviser Jeff Weaver called it Sanders’s best performance yet.

"People were energized to see him back in the fight, he gave as good as he got, and for anybody who had any lingering concerns about his health, I think they were allayed completely tonight,” he told NBC News.

Casey, the Democratic New Hampshire veteran, told NBC News that the heart attack could add a "sense of courage, renewed vigor and more purpose" to Sanders' candidacy.

"Is there a chance we'll look back and say he was falling and after his heart attack, he seemed to come around and steady the ship and maybe there's a moment he connects with voters in a different way?" she said. "I think it could go either way."