DES MOINES—Hours before the new year began, Bernie Sanders could barely speak.
"In a few hours, we're gonna be in 2016," the Vermont senator hoarsely told over 1,000 attendees at his Iowa New Year's Eve party, "And we together have an opportunity to make 2016 a year that history will long remember."
Three minutes later, Sanders abruptly wrapped his speech, quickly worked the ropeline, and briefly greeted a nearby overflow room, concluding his three day multi-event swing across the state of Iowa.
The final political event of 2015 then petered out an hour and a half early as the worn-voiced candidate left the state for home.
As the contest for the Iowa caucuses enters its final laps, experts agree that strategic pacing and stamina is vital for the success of presidential hopefuls. And with candidates from both parties ramping up their time in the Hawkeye State holding multiple multi-day swings, neglecting the specter of fatigue can be problematic.
"In presidential politics, fatigue breeds mistakes," explained David Nagle, a former state Democratic chairman and congressman from Iowa who has not aligned with any campaign at this point.
Nagle cited Vermont Governor Howard Dean's "demise" in Iowa in 2004, where a grueling schedule in the final stretch caused problems ahead of caucus night, before his now-infamous scream.
"The guy was exhausted by the time he got back to the Brown and Black Debate in Des Moines," said Nagle, referring to a traditionally Democratic forum in Iowa addressing minority issues, "He performed poorly and that performance broke his momentum."
There have already been signs of fatigue affecting candidates this cycle.
At the final Iowa event of a heavily scheduled week for business mogul Donald Trump in November, reports described the candidate as "hoarse, his hair mussed" and "tired," ultimately resulting in a "rambling" tirade that was panned as "bizarre."
At the very last stop of Senator Ted Cruz's post-Thanksgiving three-day intensive swing through Iowa, he spoke about his use of contraceptives with his wife—a remark that some evangelical leaders have not forgotten and considered a misstep, though admittedly small.
"It's better to pace yourself. When you start running a marathon that's when you trip and stumble," argued Nagle generally.
David Oman, a long-time GOP operative and Jeb Bush Iowa strategist, recalled that ahead of the 1980 caucuses, Ronald Reagan was very careful to pace himself primarily due to his age.
"His handlers were very careful. [Their thinking was] you make one or maybe two good stops a day and that's all you need to do. You're only going to get a couple minutes on TV whether you go to two places or seven," explained Oman.
Though as the calendar approaches February 1st, some candidates are taking a different approach, in what Oman acknowledges is a different era.
Cruz, the current Iowa frontrunner, will embark on a six day, 28-event bus tour through Iowa this week in his mission to visit all of the state's 99 counties before caucus night. The past two Republican caucus winners followed a similar strategy.
Former Governor Martin O'Malley began a 30-Day "New Leadership Tour" for which he has already personally phone banked and will knock on doors. Dr. Ben Carson plans to spend half of the month of January traveling the state, per his campaign.
Both Senators Marco Rubio and Sanders have gone on several three-day multi-event swings; Rubio normally hopscotching the state in a chartered plane while Sanders regularly drives from town to town.
Former Pennsylvania senator and 2012 caucus winner Rick Santorum has already boasted visiting all 99 counties this cycle.
Former Governor Mike Huckabee held the very first political event of 2016, initiating a 150-event bus tour over the course of January. This averages out to five events per day heading toward the caucuses.
"Stamina's a huge issue," Huckabee explained to NBC News as he continued to campaign with a limp after receiving knee surgery during Thanksgiving.
"Though I hate to give [other presidential candidates] any advice, 'cause I want them to lose," remarked the Arkansas Governor who was the Iowa caucus winner eight years ago.
As Cruz embarks on one of the most heavily scheduled Iowa trips this cycle, his campaign says that excitement about the final stretch of the campaign will trump the team's fatigue.
"Sure, we're all going to be tired at the end of the week," Cruz Iowa State Director Bryan English told NBC News, "But we all really get a kick out of this…and [Cruz] wants to maximize his time on the ground and he trusts the team that he's hired to execute this."
"It's a fine line between showing your voters that you're going to do everything it takes to win, and also making sure you don't run your candidate into the ground," noted former Republican Party of Iowa chair Matt Strawn, "That's a delicate balance that these campaigns are trying to walk."
"He does like to squeeze an extra [event] in here or there when he can," conceded English on Cruz's approach to the trail.
One candidate who many experts highlighted for appearing to have a balanced campaign in terms of avoiding fatigue so far? Hillary Clinton.
"She seems better paced than in 2008," remarked Nagle, recalling a tired Clinton heading into the caucuses.
Clinton has also dispatched several surrogates to both Iowa and New Hampshire ahead of the caucuses and primary, which Strawn observed is a smart way to alleviate pressure from the candidate.
Perhaps the biggest risk of overworking a candidate a month out of the caucuses experts caution, is losing one's voice, energy, or focus before a debate. Debates have become particularly important this cycle as viewership across parties contests have skyrocketed and campaigns have become increasingly nationalized.
"For the last month in Iowa…it's about making sure that your candidates do not sacrifice themselves on the altar of retail politics at the expense of the debate," added Strawn.
Despite sipping plenty of tea and honey for his ailing voice during this past swing, Bernie Sanders' campaign has said that the candidate has no intention of slowing down in Iowa at this point, noting that he does like to "micromanage" while trying to reach as many potential caucus goers as possible.
The Vermont senator has been highlighting the fact that over 35,000 people have come to see him in the first in the nation caucus state since he began his campaign.
After Sanders left his New Year's Eve event, NBC News asked attendee Danny Hull if he was concerned about the candidate's stamina in the grueling final stage of the Iowa caucuses.
"I know he's an older guy," began Hull, a retired accountant who says he will definitely caucus for Sanders, "But us older guys can take it."
Vaughn Hillyard contributed to this report.