Over the course of 2016, the nation’s economy grew by about 1.6 percent. When the American National Election Study asked people in the fall of 2016 if they thought the economy had gotten better, stayed the same or become worse over the last year, a familiar pattern emerged. Nearly half of self-described strong or weak Democrats (45 percent) thought things had gotten better over the last year. (They were of course evaluating the economy under the presidency of a Democrat.) Nearly half of Republicans took the opposite position, believing that the economy had gotten worse over the last year, while only 11 percent thought things had gotten better.

Or consider the case of Colin Kaepernick, the former San Francisco 49ers quarterback who refused to stand during the national anthem in the fall of 2016 because of what he said was police brutality toward African-Americans.

In September 2016, the HBO Real Sports/Marist Poll conducted a survey that illustrated the partisan divide: 36 percent of Democrats and 71 percent of Republicans believed that N.F.L. players should be required by the league to stand for the anthem. The controversy became more connected to partisanship when early in the season President Trump drew the nation’s attention to kneeling players, saying owners should fire players who disrespect the flag.

In the wake of these comments, HBO Sports repeated the poll. Views about whether players should be required to stand for the national anthem had polarized further. More Republicans, 82 percent in all, now believed players should be made to stand while only 27 percent of Democrats thought so. The gap between support in the two parties widened by 20 points — from a gap of 35 to a whopping 55-point spread.

Mr. Trump’s involvement in the N.F.L. protests signaled to the few people on both sides who hadn’t already sorted based on political party that it was time to apply their partisan lenses to the problem. He activated the “us” versus “them” framework, one that he repeatedly invoked in his campaign, even in a controversy that was already highly polarized. Helping people recognize that they are a part of an in-group and that people unlike them are in the out-group is part of what gives social identities power.

This is also why reactions to the recent wave of accusations about sexual harassment and assault can differ depending on who is being accused. In some cases, like the accusations against the Alabama Republican Senate candidate Roy Moore, people’s partisan group-based identities have been activated. But in other cases, like the accusations against those in Hollywood, there’s a different dynamic. While there may be some disdain among the right about Hollywood liberalism, there is no “us versus them” identity to activate — liberals tend not to see Hollywood as part of their identity.

In the case of Mr. Moore, his Alabama supporters and voters have rallied to his defense, saying that he was “innocent until proven guilty” and that the accusations are sponsored by Democrats — the “them” to their “us.” There were some similar expressions of innocence until proven guilty regarding Hollywood celebrities, but in many cases those accused have quickly lost jobs, entered therapy and often expressed regret. Democrats in Congress have so far shown little willingness to defend Mr. Franken, although President Trump has criticized his conduct.