CHARLESTON, S.C. — When Bernie Sanders took the stage Sunday evening in South Carolina, things were different.

The senator's staff usually scrambled to keep him on his busy schedule, but this was his only public event of the day. The Vermont independent and self-described democratic socialist took the advice of his aides, canceling events and appearances to go home and rest his raspy voice. He would be off the trail, but only for a day.

“I’ve been giving too many speeches lately," Sanders told NBC News on Saturday, flashing a smile, after his campaign canceled the events. "I forget that microphones work pretty well.”

The brief hiatus was a sign of the physical toll a presidential campaign takes, but it's also a reflection of the increased intensity level of his campaign as it enters a new phase.

Now, the big rallies are back. Last Monday, ahead of the Democratic presidential primary debate, upward of 10,000 people packed into a park for an evening rally in Denver, the capital of the competitive Super Tuesday state of Colorado. The campaign has rarely held rallies of this magnitude in the summer months, opting instead for smaller, more personal events targeted towards specific audiences or topics.

There has been a hiring spree — and a staff shakeup — in the early states. Sanders' New Hampshire director is moving to Massachusetts, and new campaign officials have been brought in to lead operations in Maine and New Hampshire.

Bigger, and more focused

There’s also a surge in new field staff. A hundred staff will assist the campaign in Iowa in the next few weeks, and nationwide the campaign will include well over 400 personnel, advisers say.

The boost in staff complements a shift in focus. The campaign will home in on the first four caucus and primary states, plus delegate-rich California, a senior adviser says. But this will likely come at a cost to candidate forums, featuring many candidates delivering quick stump speeches, outside those key states, another aide adds.

Campaign officials have also signaled that talks are well underway to expand early state media advertising beyond the digital space to radio and television.

The changes coincide with what political operatives consider a key marker in presidential electoral politics. Labor Day, the unofficial start of the fall season, is when many undecided and unaffiliated voters truly begin to tune in after summer vacations have ended, schools are back in session, and Americans have returned to their standard routines, political observers and operatives say.

For Sanders, whose presidential campaign four years ago challenged Hillary Clinton for the Democratic nomination, the landscape at this key point is much different — as are the resources at his disposal.

At this point in 2015, Sanders had a staff of about 40, according to Jeff Weaver who ran his 2016 campaign and is a senior adviser to his current operation. While there have been three debates so far this cycle, the first debate in the then largely unknown candidate'a previous run didn’t come until mid-October.

In that race, the campaign didn’t have a full time air charter service until after the Iowa caucuses. This time around, Sanders' team is taking advantage of that service, putting him on fewer commercial flights.

“As the campaign intensifies, it becomes logistically impossible to go everywhere we need to go without one,” Weaver told NBC News.

Aiming to broaden base

Despite the added resources, Sanders faces similar headwinds in trying to expand his base beyond his strong and active core group of supporters.

“He’s been unable for now two presidential runs to diversify his support beyond the demographics of his populist, progressive base,” said Joe Trippi, a longtime Democratic strategist who ran Howard Dean’s 2004 presidential campaign.

Sanders’ 2016 momentum was thwarted after losing in the more diverse contests in South Carolina and Nevada.

“If I were internally with them, that would be something I’d be very, very worried about in this field because there are so many other candidates who have the ability to do that,” Trippi said.

Trippi acknowledged the campaign's efforts to fix the issue through an emphasis on outreach to minority voters, and campaign aides said they think Sanders is making progress. The campaign — and sometimes the candidate himself — regularly touts polling to suggest the strategy is working, including a pre-debate polling memo that they say showed Sanders leading nationally with Latino voters in the crowded primary field.

The memo included recent polls from CNN and Univision showing Sanders about even with former Vice President Joe Biden in support from Hispanic voters. Among African Americans, Sanders trails behind Biden, but he is doing well with younger black voters, with one poll showing him running about even with Sen. Kamala Harris of California in support among young black women.

The campaign's efforts also show in its schedule. Nevada and South Carolina were the first two states Sanders campaigned in after the September Democratic debate.

In Las Vegas, Sanders led an invitation-only town hall focused entirely on “Latinx issues.” In recent weeks, he’s hosted topical events for union workers, on climate change and on Medicare for all — issues the campaign says appeals to broad demographics.

Despite having to cancel appearances at several South Carolina events to rest his hoarse voice, Sanders is already scheduled to return to the South next weekend for events at three historically black colleges — another post-Labor Day change.

As students begin the fall semester, Sanders is taking advantage of the high concentration of young people on college and university campuses, kicking off campus tours this month aimed at a demographic group that the campaign is relying on in order to succeed.

Speaking to NBC News at a Labor Day parade, Sanders acknowledged an increase in the intensity of this race, but he believes he has a key advantage.

“We have a lot of grassroots energy. We have a lot volunteers,” he said, standing in front of a group of volunteers marching with him in Milford, New Hampshire. “What really matters is voter turnout, what matters is the kind of enthusiasm and energy you have, and I'm feeling pretty good about that.”