MANCHESTER, N.Y. Bernie Sanders, just weeks after a heart attack took him off the presidential campaign trail, reopening questions about his age and health, roared back last week with a strong debate performance and the disclosure of a quarterly fundraising haul that defeated all of his democratic rivals.

But the 78-year-old Vermont Senator, whose powerful oratory and progressive message on income inequality raised him to serious contention in the 2016 Democratic fight against Hillary Clinton, is less formidable this time around, with polls in early States and beyond showing his status as a top-tier candidate in jeopardy.

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

Aside from Tuesday’s televised debate in Ohio, Sanders has been largely off the trail since his October 1 heart attack. He held his first major campaign event since his hospitalization on Saturday, when new York Republic’s Alexandria Ocasio-Cortez joined Sanders at a rally in new York to endorse his candidacy.

“I’m more than willing to take on the greed and corruption of the corporate elite and their apologists,” Sanders told thousands of supporters in Queens. –I am more than ever ready 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’m back.”

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 the 1.05 million Trumps.

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

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

Sanders ‘ poll numbers, which have begun to slide in recent months as Warren has taken steam, showed a steeper dip after news of his heart attack was revealed. The 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 biggest challenge to the Sanders campaign has been lingering disagreements about how to refit the strategy from his 2016 campaign to 2020. Those clashes were particularly evident 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 recent state polls put him in third place, trailing Warren and Biden by double digits even before his health scare.

“He’s the only one who fell like a stone. From high polls, and high expectations, he was brought back to earth, ” says 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 position as head of operations in Massachusetts. Around the same time, Sanders ‘ senior adviser and new Hampshire steering Committee member Kurt Ehrenberg tore up an exceptional campaign.

“A clear, clear sense of the pervasiveness of 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 the staff shake-up was announced last month.

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Some staffers also left the campaign in Iowa, the state’s first-in-the-nation Caucuses. A recent Des Moines Register poll showed Sanders in third place in the state, again behind Warren and Biden. A post-heart attack poll of 18 early States through Super Tuesday March 3 showed similar results.

From the outset, the Sanders campaign has faced the challenge of managing expectations in a diverse, crowded field of fresh faces this year.

And instead of expanding his loyal base since 2016, Sanders has struggled to maintain it, as many of his 2016 supporters are less enthusiastic about his runoff.

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

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

Current and former Sanders aides acknowledge that Warren is ahead of them, both in message management and campaign organization in early States. But they also point to policy proposals they say go much further than Warren, like a more robust Green New deal to combat climate change and a more aggressive wealth tax.

Aides also point to differences between the two candidates that could work for Sanders in the future, including a more diverse base of supporters.

An NBC News/Wall Street Journal POLL conducted in September found that support for Warren is strongest among liberal white College-educated 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 now polls show that black and less educated voters still favor Biden over Sanders.

“He’s currently squeezed on both sides, one Warren and one Biden. And he just doesn’t make a compelling case for why people should go back to him, ” a senior democratic strategist familiar with the campaign told NBC News.

At the recent opening of a field office in Dover, new Hampshire, campaign co-Chairman Ben Cohen of Ben and Jerrys Ice Cream said the reaction inside the campaign to Sanders ‘ heart attack was one of activism, not concern.

“It was kind of a rallying cry. People are more mental, more motivated than ever, ” he told NBC News. “Personally, I had a quadruple bypass surgery on an open heart. It’s a big deal. This guy had some little stents. It’s not a big thing.”

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

“We’re way ahead of where we were four years ago, in terms of staffing, in terms of operations,” state communications Director Michael Wukela said, adding that 72 percent of Sanders employees in the state are people of color.

Still, many voters who supported Sanders in 2016 say they went further.

Iowa city resident Dawn Harbor caucused for Sanders in Iowa in 2016, but said she’s leaning heavily toward Warren this time around.

“He’s much more angry. She’s a lot more balanced, ” Harbor said. “I mean, Bernie was doing the running forever and we wouldn’t be here without him, that’s true. But at this point, we need her.”

And for other voters, Sanders ‘ health is now before the mind.

Anya Blanchard, 19, told NBC News in the senators ‘hometown of Burlington, Vermont, that she was still undecided, but that Sanders’ heart attack had given her pause.

“It kind of makes me not want to vote for him as much just because I want someone in office who will not be having to focus on his health as a top priority,” Blanchard said.

Still, Sanders ‘ nationally televised debate performance last week was strong and may have assuaged the concerns of some supporters. After the debate, senior campaign adviser Jeff weaver called Sanders the best job.

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

Casey, a veteran new Hampshire Democrat, told NBC News that a 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’s falling and after a heart attack, he seemed to come and steady, and maybe there’s a moment where he connects with voters in a different way? She said. “I think it could go either way.”