NEWTON, Iowa — Riding through Iowa in Joe Biden's "Soul of the Nation" bus can be a disorienting experience. The same solid blue exterior that makes his rolling campaign headquarters conspicuous on the road makes it nearly impossible for those inside to see anything outside. One passenger this week described it as similar to being on a dark airplane, shades down, endlessly taxiing but never taking off.

It's a sensation that only deepens the feeling of uncertainty about the campaign's future. Nine months after Biden launched his third White House bid, his campaign is well aware of how the trajectory could change in an instant at the caucuses here Monday night — a strong finish accelerating his path to the Democratic nomination or a disappointing finish suggesting the beginning of the end of a distinguished career in elected office.

In an exclusive interview with NBC News this week as he rode between stops, Biden himself remained upbeat about his chances here even as he stressed that it's just the first step of what he expects to be a long nomination fight.

"Nothing happens here on Monday's gonna end this campaign," he said. "I mean, I'd rather have an outright win, don't get me wrong."

But he said the most likely outcome in Iowa is a tightly bunched field. And in New Hampshire, he said, Bernie Sanders and Elizabeth Warren have a neighboring state advantage.

"I think I'll do well in Nevada," he said. "And I think I have a real firewall in South Carolina. And then we go into the Super Tuesday states that have a significant number of minorities and African Americans," where "I think I'm gonna do fine. So I don't think that this is like it has been in the past, that if you haven't won the first two that you're done."

In the closing week here, the Biden campaign has sought to maximize the advantage of having one of the few front-running candidates not tied up in Washington for President Donald Trump's impeachment trial. His bus logged more than a thousand miles as part of a nine-day closing blitz of the state, with 23 events taking him from Des Moines to the Mississippi, across the state to the Missouri River and back again.

Each campaign stop along the way is filled with more than the hour Biden spends on stage or the half-hour on the rope line greeting supporters in a blur of selfies, handshakes and hugs. There are also local media interviews, private meetings with volunteers and local officials and, more often than not, a cluster of traveling reporters staking out his entrances and exits.

Even the bus is no reprieve from the frenetic pace. There's staff time in the cramped rear cabin to keep him informed about his operation, update him on campaign and news developments elsewhere and prep him for the next event.

Closer to the front, there are phone calls to make — on Thursday, it was to thank a pair of precinct captains and to show solidarity with union allies on a hunger strike.

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"The calls are kind of the hardest thing to get in," Biden said as he prepared to call Miami airline catering workers.

At another point, when Biden looked up from his briefing book on the drive between rallies in Newton and Ottumwa, he also managed to catch a glimpse of a familiar logo on the road ahead.

"Is this Dairy Queen?" Biden asked his aides. "I just saw a Dairy Queen."

Staffers responded that they were looking into stopping at another of the ice cream chain's locations farther down the highway.

"No, don't look into it. Stop," he ordered in jest but in a way that made clear what his preference was. "This isn't looking into it."

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Even in the cold Iowa winter, it's a Biden campaign ritual to stop for ice cream whenever possible. His bus made a brief detour Thursday to a DQ in Pella, where Biden ordered Blizzards and sundaes for his bus driver and staff, a stop that also yielded his first extended question-and-answer session with reporters in over a month.

Biden was asked about some of the new attacks from his rivals, this time Pete Buttigieg. The question touches on what he says is his biggest concern at this point — that as the elbows have grown sharper in the closing stretch, it will only grow harder for the party to unite behind the nominee.

"I think it's a big mistake — and I've tried not to counter — for Democrats to go after one another this time around. We have enough disagreement on substance without misrepresenting records," he told NBC News earlier. "But the interesting thing is, if you'll notice, it's usually the person who attacks who is the one who gets hurt by the process."

Biden joined the contest late, and as a fragile front-runner — a familiar and well-liked figure in the Democratic Party but one whose liabilities were just as well known — he was quickly seized on.

Even before he formally joined the race, there was a former Nevada lawmaker speaking out about what she considered inappropriate physical contact from Biden years earlier. From there, he has endured a seemingly never-ending series of negative news cycles that kept his campaign, and at times the candidate himself, constantly playing defense.

And then there's the fact that Trump was impeached for a scheme attempting to smear Biden, which meant he was never far from that conversation, as well. While Biden blitzed Iowa, the president's impeachment managers continued to promote a debunked narrative that, as vice president, Biden exerted pressure on the Ukrainian government to protect his son Hunter, who was serving on the board of a natural gas company there.

Still, Biden has — at least according to polls — as good a chance of any candidate to win the nomination.

"I've been the only one having — until recently — having a lot of punches thrown at me across the board," he said. "But virtually everything that's been thrown at me, none of it has stuck."

"The vast majority of the American people know me," he added later. "The good news is they know me. The bad knows is they know me. You know, the attacks on me and my weaknesses are real, but the attacks on my strengths just aren't goin' anywhere."

That doesn't mean Biden's supporters don't want to see a more forceful response to the president's continued assaults over the Ukraine issue, among others.

"I kind of do wish Joe would come out more forcefully and say: 'Maybe it wasn't a good decision, but he's a grown man. I'm his dad, but I'm not the one that makes his decisions for him,'" said Julie Haindfield, an Iowan sitting in the front row for a Biden event in Sioux City who said she would caucus for him Monday. "You know, maybe Joe should say, 'In retrospect, maybe Hunter shouldn't have done it, but it's over and done, it's been investigated, he did nothing wrong,' and go out more forcefully about that."

An otherwise relaxed and reflective conversation with Biden on his bus turned more stern when he was asked whether he should more directly rebut the charges against him and his son.

"Well, first of all, there are no charges," Biden said. "It's kind of fascinating. The charge is he was on a board. No one's ever accused him of doing anything wrong on the board."

At a stop in Council Bluffs, the first question Biden fielded wasn't a question but a plea not to let Trump "bully you." But in the interview, Biden made it clear that he won't get lured into daily tit-for-tat battles with the president.

"The only place he's comfortable, it seems in the past, is in the dirt, you know? And there's so many more things that are consequential," Biden said. "One of the mistakes made in the last election is he was able to pull the party into a debate about things that didn't relate to the issues. Because he doesn't know much about the issues. He doesn't. That's why I'm anxious to debate him."

Asked whether he'd have any regrets about running if he were to lose the nomination, Biden quickly answered no — before turning to a solemn place and his late son, Beau.

"I made a commitment to Beau, not that I would run, but that I would not back away from things I've always done and cared about. And so there is no regret," he said. "I will have as long as I do my best and, as Beau would say: 'Dad, remember home base. Remember who you are. Just be who you are and make your case.' If my case can be made in a way that people accept it, all the better. If they don't, they don't."

Twice in this campaign, Biden and his wife, Jill, have said the person in the family they expected to be running for president right now was Beau. "If he had run and won, he'd be a 50-year-old, 51-year-old president," he said. "I wouldn't be doing this, were it not Trump."