Why was the period from the late nineteenth the mid-twentieth centuries called "the golden age of parties"?

Answer: There are many reasons for that:

There was party stability, and the dominance of both Democrat and Republican parties was strong, which led to a big impact in the lives of millions of voters. The appearance of political machines, which were party organizations that incentivized people to join their party and increased their voter's loyalty. Political machines became a central element of life for civilians. Political parties not only served the political needs of society, but also their needs for social services.