In the first half of the 20th Century, one of the deepest fault lines in American politics divided labor unions from manufacturers. They constantly fought over who got the bigger piece of the pie.
Yet, these political enemies also united to back one major policy: tariffs. They understood that they had a good thing going together here in America, so why let outsiders horn in on the deal? It was in their interest to agree 5% of the time so that when they fought the other 95% of time they'd both come out better off.
This same old-fashioned logic suggests that American whites and American blacks would be wise to unite politically to restrict immigration. Whatever else you can say, on the whole, we've all got a good thing going together here in America, so why let outsiders horn in on the deal? We can get back to fighting over how to divvy up the pie afterwards.
But, this kind of common-sensical single-purpose political alliance seems much harder to pull off these days, when globalist interests control most of the vocabulary on all sides of the fence. Now, it's far more morally respectable for globalists of varying hues to put together an enormously cynical coalition to increase immigration.