There are technically more than two political parties, but only two get the attention and the money to win elections from time to time.
The Democratic Party was founded by Thomas Jefferson. For a long time after the Civil War it was a predominantly Southern party, until Franklin D. Roosevelt refashioned it into a coalition of groups that benefited from measures to combat the Great Depression in the 1930s. That is when the Democrats first became liberal-leaning and favoring labor unions and worker rights, later civil rights.
The Republican Party was founded by Abraham Lincoln. It arose out of many of the issues that gave rise to the Civil War and was supported mostly by the Northeastern elites made rich by industrialization. For much of the second half of the 19th century it was a majority party thanks to that support (and the exclusion of many groups from voting). The FDR coalition, which fell apart in 1968, turned it into a minority party until the 1980s. The Grand Old Party, as it was called long ago, reversed its fortunes primarily by attracting the votes of white
Southerners who never forgave the Democratic Party for supporting the Civil Rights Act of 1964, which ended legal segregation.
At present, the long-term demographics are working in favor of a future Democratic majority, as non-Hispanic whites are rapidly becoming a minority (Anglo-Saxon whites already are), but the plutocracy may find a way to corrupt the political process and keep the Republicans in play.
This is a repost from my replies to questions posted on Quora, a question-and-answer site where questions are asked, answered, edited and organized by its community of users, at quora.com. The questions and their subtexts are not mine.