And yet I know many Catholics who are married to divorced people.
That is true. However, technically, if the first marriage was not annulled, a Catholic priest is not allowed to marry them again. In fact, having an unbiblical divorce forbids people from receiving communion in the Catholic Church.
I also find it common that many people who call themselves "Catholic" do not actually practice the faith at all. It is used more as a title or identity statement. However, in all fairness, this is true for other denominations as well. My point is that just because somebody calls themselves "Christian" or "Catholic," doesn't mean they actually follow the teachings of Christ. It only means they recognize the existence of God, but nothing else. After all, James said, "You believe in one God, you'd do well. But even demons believe in God, and tremble." This, in my opinion makes these carnal Chriatians worse than demons. Because at least Demons have a holy fear of the Lord.
I also want to clarify that just because someone practices the Christian faith horribly, it doesn't mean they are "not real Christians" or that they are not saved. Only God can judge. Because Jesus did not come to the earth to make bad people good, he came to make dead people alive.
So, to bring this back on topic, it isn't fair to say whether or not Biden or Trump are not real Christians because they are both have a history that goes against Christ's teachings. It means that, just like all of us, they are imperfect humans who are saved by grace, not merit.