I've never really understood why evangelical Christians in the US ally themselves with the Republican party.
This is a really complicated thing, because there have been changes over time, but a lot of people don't realise that until abortion was a hot-button issue in politics, the Democrats were the party of many Christians, especially Catholics, but also some others. In a real way the Republicans used that issue, which was so fundamental to many, to get those votes even though they were not aligned in other ways. Over time many of those people began to associate themselves more with the Republicans and so their other political views shifted.
Evangelical Christianity in the US though is very much an American invention, its a radical individualist interpretation of Christianity that quietly (or not) dropped many of the elements that emphasise collectivity in mainline Christianity. The American approach to secularism is also different to the British or European approach and tends to favour religion as an individualist endeavour rather than something with a social element.