Roosevelt's implementation of his popular New Deal expanded the size and power of the federal government to an extent unprecedented in American history, and marked the beginning of political dominance by the Democratic Party that would remain largely unbroken until 1952.

[2] Despite the power of the New Dealers, the conservative coalition, comprising northern Republicans and southern Democrats, generally controlled Congress from 1938 to 1964.

Although still broadly consisting of the White Anglo-Saxon Protestants who dominated the conservative coalition as well, New Dealers also grew to include new ethno-religious constituencies, such as Catholics and Jews, in addition to liberal White southerners, trade unionists, urban machinists, progressive intellectuals, populist farm groups, and even some ex-Republicans from the Northeast.

[4] The Republican Party underwent a dramatic ideological change of its own during this period, consisting of a conservative wing led by Senator Robert A. Taft and then Barry Goldwater, and a moderate to liberal wing led by Thomas Dewey, Nelson Rockefeller, Earl Warren, Jacob Javits, George W. Romney, William Scranton, Henry Cabot Lodge Jr., and Prescott Bush.

He notes that, "There seems to be consensus on the appropriate name for the sixth party system... Changes that occurred during the 1960s were so great and so pervasive that they cry out to be called a critical-election period.

"[15] The Princeton Encyclopedia of American Political History dates the start of the Sixth Party system in 1980, with the election of Reagan and a Republican Senate.