I do not believe that most church congregations in the US are "segregated", at least not on racial or ethnic lines.
Segregation involves a level of enforced separation.
(Others have already spoken to some of the churches that do separate worshipers based on gender).
Why do some church congregations tend to be predominantly white, predominantly black or even predominantly brown (Hispanic)?
I think someone in the thread earlier mentioned that congregations often are made up of people from the neighborhood around the church. And in many parts of the US neighborhoods may be made up of people of the same color or ethnic group.
Furthermore, some Christian churches in the US had a history of congregations based along ethnic lines. I grew up in Chicago. A hundred years ago you might have 3 Roman Catholic churches within 2-3 blocks of each other---you'd have a "Polish" church, a "German" church and a "Bohemian" church. In Chicago you had German Lutheran churches, Swedish Lutheran churches, Norwegian Lutheran churches, etc. Immigrants from Europe came and settled in neighborhoods and socialized with their neighbors and people that they were familiar with. You may still have that along some racial/ethnic lines but not to the extent that it existed 100 years ago. Today in Chicago you will find some Roman Catholic churches whose congregations are primarily Hispanic and who conduct their services in Spanish.
There are also some religions whose congregations are made up of predominantly the same ethnic group (Buddhists are predominantly Asian for example) but that (I believe) is more due to the fact that Buddhism originated originated in Asia.
Are there still Christian churches that are predominantly white, predominantly black, or predominantly Hispanic? Sure, but I don't think that it is anything that is enforced. It's not due to segregation.
As for
I would say it was prompted by the rising air of Christian bigotry being generated by the rampant rhetoric of the Republican presidential campaign.
ALL politicians appeal (or some may say pander) to their constituency groups.
You have some Democratic candidates that speak at churches, meet with church leaders, etc., just as you have some Republican candidates that do the same thing. That you may see more Republican candidates appealing to Christians is not surprising. It's constituency politics.
You also have 2 frontrunner Democratic candidates at the moment. You have what, close to a dozen Republicans still running.
That means there are going to be more occurrences of Republicans appealing to this constituency.