About three weeks ago — before President Trump said that N.F.L. owners should fire players who kneel during the national anthem — Democrats and Republicans held relatively similar views about the league. About 60 percent said they viewed it favorably, while about 20 percent said they viewed it unfavorably, according to daily online surveys conducted by Morning Consult, a polling, media and technology company.

Since Mr. Trump’s remarks, though, many of his supporters have changed their attitudes.

Trump voters are now much more likely to say that they view the N.F.L. negatively, reflecting a sharp change around Sept. 23, when Mr. Trump criticized the players at a speech in Alabama. The views of Hillary Clinton voters have not changed appreciably over the last few weeks.

Some of the difference may be a result of our collective filter bubbles, which make Americans more likely to engage with and share articles that reinforce their views.

Both Democrats and Republicans were more likely to report seeing negative news about the N.F.L. around the time of Mr. Trump’s remarks in Alabama and the protests that followed. But Trump voters were much more likely to do so, while respondents who said they voted for Mrs. Clinton were more mixed in the coverage they reported seeing.

One possibility is that the two groups are not reading the same kinds of articles, avoiding those they disagree with. Another is that people are simply taking different attitudes away from the same articles, tweets and Facebook posts.

The shifts are not limited to professional football. After LeBron James tweeted his support for Stephen Curry, whose invitation to the White House was rescinded by Mr. Trump (after Mr. Curry indicated he wouldn’t attend), the share of Trump voters who said they held very unfavorable views of Mr. James more than doubled, to 23 percent from 11 percent. For Clinton voters, the opposite was true: The event made them like Mr. James more.

In other polls, Americans’ views on the N.F.L. protests depend largely on how the questions are asked, and whether they emphasize patriotism, free speech or race. And while N.F.L. ratings are down compared with previous years, protests are probably not the main reason.

A helpful way to consider Americans’ attitudes toward the N.F.L. and the N.B.A. is to compare them with their views on other things. Every day, Morning Consult conducts large online surveys of about 5,000 adults in the United States, asking respondents their opinions about hundreds of brands and companies.

Most of the time, ratings are consistent among people of different political parties. For example, Americans have favorable views of Oreos, Home Depot and Bed Bath & Beyond, regardless of who got their vote in 2016. (And, on the whole, both Trump and Clinton voters have slightly negative views of companies like Goldman Sachs and Philip Morris.)

But a tiny fraction of these companies are divisive, with politics shaping how Americans perceive them. The N.F.L. has moved near the top of this list in recent weeks:

The Most Polarizing Brands and Companies Measured by Morning Consult

The N.F.L. is now among the nation’s most divisive brands, behind Trump Hotels and a handful of media companies, including The New York Times.

It’s hard to know if or when Americans’ views of the N.F.L. will go back to how they were before these protests. President Trump has continued to fuel the controversy, directing insulting tweets this week at Jemele Hill of ESPN; the N.F.L. players who protested; and the league itself. On Sunday, Vice President Mike Pence walked out of an Indianapolis Colts game when some players on the visiting team knelt during the national anthem. (President Trump said he directed Mr. Pence to do so.)

But a lesson of outrage in 2017 is that its lifespan is always decreasing.