Operators of two of the largest U.S. ticket marketplaces say they are seeing declines in orders for NFL games amid festering controversy over the national anthem.

NFL ticket sales at TickPick slumped 17.9 percent this week compared with the prior week, their steepest decline since 2014, while sales at TicketCity plunged 31 percent. For pro football games in September, ticket sales at TicketCity are down 16 percent from a year ago. TickPick's NFL sales rose this month, but by the smallest margin since 2015. The vendors' data reflect tickets sold in the secondary market, as opposed to direct sales from NFL teams.

The slump in ticket sales underscores the pressure facing the NFL over the anthem protests, which were started last year by former San Francisco 49ers quarterback Colin Kaepernick. The league's television ratings also appear affected, with viewership down 11 percent so far this season and some fans are calling for a boycott of the NFL's corporate partners.

"There are people who are saying that this is my form of entertainment and I don't want politics in my entertainment," said TickPick co-CEO Brett Goldberg, emphasizing that he doesn't personally share that viewpoint, in an interview.

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A new poll by CBS News found that Americans are unhappy with both the protests during the anthem and President Trump's comments on the issue.

Views hinge on partisanship, but also shift with how Americans view the intentions, not just the actions, of the players and president. Most Americans say that the players' intention is to try to call attention to racism. However, Republicans also feel the players are trying to disrespect the flag and the military, too – and strongly disapprove.

According to Mike Guiffre, TicketCity's vice president of sales, NFL ticket demand has also been hurt by lackluster games and the league's ineffective marketing.

"They have not effectively branded, priced, marketed or sold tickets in a modern enough fashion and have counted on season ticket sales and corporate seats for far too long," he said in an email. "It drove the fans away slowly."

Ticketmaster, which operates the NFL's official ticket exchange, couldn't release any sales information about the league without its permission, said Ashley Dos Santos, a spokeswoman for the Live Nation-owned business. The NFL couldn't immediately be reached.

StubHub, which didn't respond to request for comment, is offering a 10 percent discount on NFL, NBA and NHL tickets in a promotion on its website, a tactic Guiffre said is highly unusual for the ticket service.

Attending NFL games isn't cheap. According to Vivid Seats, tickets cost an average of $172 this season, an increase of 6 percent compared with last year. Fans face wait times that can stretch into the decades for the right to buy season tickets for some teams.

The latest kerfuffle over the anthem began when President Trump last week rescinded the invitation of NBA champions the Golden State Warriors to visit the White House after the club's star player, Stephen Curry, said he was uneasy about the visit.

Although many in the sports world condemned Mr. Trump's remarks as tone-deaf, he then urged NFL owners to fire players who "disrespect" the flag by not standing during the anthem. Several owners, including Jerry Jones of the Dallas Cowboys and the Philadelphia Eagles' Jeffrey Lurie, stood in solidarity with their players.

The Green Bay Packers asked their fans at Lambeau Field in Green Bay, Wisconsin, to join them in locking arms in solidarity during last night's national anthem prior to their game against the Chicago Bears. Some, however, chose not to participate.

Even with the controversy, coupled with mounting public attention on the link between football an brain injuries, the NFL remains a financial powerhouse. The league remains by far the most popular U.S. professional sport and is expected to generate $17 billion in revenue this year. Even with its recent ratings decline, its games also among the most watched shows on television.