@augustus the Republican party developed around the time before the Civil War and was generally pro-northern, which is why Lincoln was a republican, while the Democratic party came from an older, initially populist/rural party during Andrew Jackson's time (originally the Democratic-Republicans iirc)

after the war, the Republicans became the party of big business and the north, while the Democrats remained southern and a little friendlier to workers, farmers, and urban immigrants (due to political machines that basically used immigrants as voting fodder) which is ultimately how FDR was able to use its voter base to turn it into a sort of democratic socialist party

during the early Cold War things got confusing (at least to me); I think the Republicans were generally more anti-communist, hence guys like McCarthy

then Nixon's campaign guys were able to turn the Republican party into a sort of advocate for the South, because the Democrats had shifted away from the South during the mid century, and Reagan continued that by e.g. focusing on moral issues and courting Evangelical Christians, and you probably know the more recent history thereafter

(to other red-blooded Americans: feel free to correct me if you find any inaccuracies)