U.S. Sen. Bernie Sanders returned to Iowa on Thursday to discuss corporate corruption at a series of events across the state.

It was Sanders' first visit since suffering a heart attack earlier this month and canceling several events. He told the Des Moines Register in an interview that despite the cancellations and the decision to scale back the pace of his schedule, his campaign has not lost momentum at this crucial point in the race.

"I will tell you, what was very emotional for me is that, when I had the heart attack, what we saw all over this country was people say, 'We're going to stand with Bernie,' " he said. "And they got even more involved."

At Sanders' first event of the night, a town hall in Marshalltown, the audience was widely unconcerned by the Vermont senator's health.

"I've seen people go through heart attacks and come through it ... in better shape," said Sandra Nodland, a 69-year-old retiree from Marshalltown. "I'm not worried about that at all."

For Marshalltown high school teacher Alex Amme, however, the heart attack cemented fears that Sanders is too old to serve as president.

"I think the age concerns me, and then the heart attack puts me over the edge," said Amme, age 32.

Sanders addressed these concerns directly in his speech Thursday night.

"I've been criticized for being old. I plead guilty: I am old," Sanders said, to cheers and laughter from the audience. "But there are some advantages to being old. That is, when you have been involved in public life for many, many decades, you and the people of this country have an opportunity to take a look at my record."

His speech centered on themes of corporate corruption and wealth inequality in America. Sanders openly named companies like Exxon Mobile and Amazon, accusing them of lying about climate change and evading income taxes, respectively.

He also touched on the pillars of his campaign: raising the minimum wage, instating a "Medicare for All" health care system and eliminating student debt, among others.

The event was small compared to most Sanders rallies, with only about 130 people in attendance, but the audience was enthusiastic. Throughout his stump speech, Sanders posed questions to the crowd, asking them to shout back answers.

There is one week until the Iowa Democratic Party's Liberty and Justice dinner — a key event that has acted as a turning point in the sprint to Caucus Day — and there are just over three months until the caucuses.

But Sanders' campaign worked to maintain an Iowa presence, even in the senator's absence, bringing in surrogates like U.S. Rep. Ro Khanna of California and airing television ads.

On Friday, Sanders plans to attend a town hall focused on Latino voters in Newton and host events in Toledo and Iowa City.

► More:Iowa Caucus Candidate Tracker: See where the 2020 candidates will be

In Iowa City, Sanders will be introduced by his latest Iowa endorser, influential Democrat and Linn County Supervisor Stacey Walker.

Sanders also recently secured the coveted endorsement of U.S. Rep. Alexandria Ocasio-Cortez of New York. The freshman congresswoman appeared with Sanders at a rally in New York City that drew an estimated 26,000 people.

"To have leaders in the community who are respected by other people in the state, certainly, is a positive," Sanders told the Register.

He said the endorsements and the massive rally in Queens, New York, reflect the overall positive position of his campaign.

However, the latest Des Moines Register/CNN/Mediacom Iowa Poll showed him slipping from second to third place behind U.S. Sen Elizabeth Warren of Massachusetts and former Vice President Joe Biden.

"Today, your poll is good; tomorrow, it's bad," Sanders said, dismissing the often up-and-down nature of presidential polling. "At the end of the day, especially in a caucus state, the results will be determined by one factor, and that is voter turnout. And I think we have the capability of bringing out our supporters in very large numbers, and I think we stand a great chance to win here in Iowa."

Sanders did not say he considers Iowa a "must-win" state as he seeks the Democratic nomination for president, but he said it is important for any candidate to do well here. Sanders came within a fraction of a percentage point of beating eventual Democratic nominee Hillary Clinton in 2016, and he went on to win in New Hampshire shortly after.

"We have, right here in Iowa, I believe, more volunteers than any other campaign," he said. "I think we have more campaign contributions coming in from Iowa than any other candidate. So we have a lot of grassroots support."

Brianne Pfannenstiel is Chief Politics Reporter for the Register. Reach her at bpfann@dmreg.com or 515-284-8244. Follow her on Twitter at @brianneDMR.

Support reporters who follow your government by becoming a subscriber. Sign up at DesMoinesRegister.com/Deal.