The NFL season kicks off en masse Sunday on the 15th anniversary of the Sept. 11 attacks, and the playing and observance of the national anthem — once considered a symbol of national unity — will be in the spotlight as America wonders if additional players will choose to kneel, instead of stand, to protest racial divisiveness.

Colin Kaepernick, whose San Francisco 49ers don’t play until Monday night, triggered what could be a growing movement among players on Aug. 26 when he chose to kneel before the national anthem at a preseason game against the Green Bay Packers. On Thursday, Denver’s Brandon Marshall knelt on one knee before his team’s season opener and the entire Seahawks team is said to be considering either sitting or kneeling before their game Sunday against the Dolphins.

“People don’t realize what’s really going on in this country,” Kaepernick said of his decision not to stand. “There are a lot of things that are going on that are unjust. People aren’t being held accountable for. And that’s something that needs to change.”

There is no rule that players have to stand for the national anthem. But there is a clear divide among NFL players, fans and executives when it comes to whether or not they should and what it means if they don’t. Are players un-American for not standing? Or are they patriots for exercising their First Amendment rights?

On his WFAN radio show, Boomer Esiason called Kaepernick “about as disrespectful as any athlete has ever been.” Jets wide receiver Brandon Marshall disagreed strongly.

“This guy, he’s one of the biggest patriots out there,” he said to Boomer and his radio partner, Craig Carton. “Because he’s standing up for human rights.”

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NFL commissioner Roger Goodell seems to come down somewhere in the middle.

“I don’t necessarily agree with what he is doing” Goodell told The Associated Press last week. “I support our players when they want to see change in society, and we don’t live in a perfect society. “On the other hand, we believe very strongly in patriotism in the NFL. I personally believe very strongly in that.”

Even President Obama weighed in on Kaepernick’s protest.

“When it comes to the flag and the national anthem and the meaning that holds for our men and women in uniform and those who fought for us, that is a tough thing for them to get past,” Obama said. “But I don’t doubt his sincerity. I think he cares about some real, legitimate issues that have to be talked about. If nothing else, he’s generated more conversation about issues that have to be talked about.”

DIVERSE LOCKER ROOMS

Considering the divisiveness of the current presidential election, it shouldn’t be such a surprise that politics has found its way into football.

In many ways, an NFL stadium is a microcosm of America: It is a place where the son of a cattle rancher from Texas snaps the ball to a second-generation American from New Orleans, who throws the ball to a wide receiver whose mother was born in Puerto Rico. It is a place that is both racially, geographically and politically diverse. Florida California, Texas, Georgia and Ohio — a representative mishmash of blue, red and swing states — produced half of the NFL’s players in 2015, according to a roster tally in USA Today.

“I grew up in Arizona where everyone was kind of the same,” Jets quarterback Ryan Fitzpatrick said. “One of my favorite things about playing in the NFL is how diverse the locker room is. It’s not ethnicity or religious backgrounds. It’s socioeconomic backgrounds. I’ve gained a better understanding of the United States and how it works just listening to guys’ stories and upbringing and things they went through in their childhood.

“The one common thing we all have is football and our love for the game of football. It’s what unites us.”

The NFL is also a place where it is critical to success that everyone is on the same page. This has been lost on coaches and players who are looking for ways to keep their team together during an election cycle and political climate that has divided so much of the country.

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“We talked about it quite a bit in our first meeting when we got together,” Giants coach Ben McAdoo said during training camp. “We talked about how outside of the building there is an election going on, a lot of racial tension, sexual assaults happening, tension with law enforcement, terrorism. We kind of live in that bunker mentality in our world and when we get a break and go outside, it is somewhat disturbing. I wanted to make sure that in the room, we’re all family.”

DIFFERENCES OF OPINION

Giants wide receiver Victor Cruz, who stumped for Obama and said he is a Hillary Clinton supporter, said he sometimes has a hard time when he finds out someone is supporting Donald Trump. On Aug. 30, Trump called Kaepernick’s protest “a terrible thing.” adding that “maybe he should find another country that works better for him.”

Some NFL players found the remark — and previous comments made by Trump — offensive.

“Obviously, you think about how they view life to support someone who feels like someone of Mexican decent, of Hispanic descent, shouldn’t be allowed in this country,” Cruz said. “It’s difficult. You don’t want someone that closed-minded leading your country. The people who follow him? Well it’s tough to think about his morals and values and now I have to look at them and in a way and wonder what are your morals and values.”

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Jets coach Todd Bowles hasn’t formally addressed the issue of the divisive political campaigns with his team.

“I know everyone has a unique background and different beliefs, but when the team comes together our focus is on football,” Bowles said. “If there’s something that needs to be addressed, it is.”

Jets center Nick Mangold has stumped for Trump on the campaign trail in Ohio. Jets owner Woody Johnson is one of six joint vice chairman of the Republican fundraising committee formed in May to finance Trump’s race.

This week, Johnson was asked what his reaction would be if a Jets player chose to join Kaepernick in not standing during the national anthem. “It wouldn’t be my first choice,” Johnson said. “I would have a conversation at that point with him to try to find out what is motivating that person to do that.”

Jets defensive end Sheldon Richardson said players are more likely to argue about basketball teams and music than they are about the election. Politics, however, do get discussed.

“You vote for who you vote for,” Richardson said. “Guys might crack a joke here or there or say something different. Guys might have a comeback for them. But we’re still a team. We still have a bigger goal than what your personal beliefs are.”

Giants running back Rashad Jennings believes the locker room is a perfect place to have a political discussion as long as it can be done in a civil manner.

“I am that guy, the guy who brings up uncomfortable topics and issues,” Jennings said. “I give my opinions to try to find common ground. Understanding someone’s point of view is pivotal to me to build a relationship with someone else. And you need to have great relationships to build championships. Topics do come up. I think it’s good to have a mediator whose job is to bring it back to common ground. Then you can have real conversations.”

McAdoo also believes it is important to have conversations and try to understand points of view that aren’t your own. And that is why the first-year Giants coach decided to address the topic on the first day of training camp.

“I had to get the elephant out of the room and let them know that I had never walked in the shoes of a black man or woman or police officer or a religion other than my own,” he said. “But it is a decision to choose empathy over violence, understanding over judgment. . . . .This league is a great platform. And my message is that we can make a difference.”