I can think of two possible origins of "progressive" as an ideology. First, Theodore Roosevelt’s “Progressive Party,” which was founded on egalitarian principles in 1912. The second comes from economics: a “progressive” tax is a tax scheme that burdens in the wealthy more heavily than a proportional tax. These two uses are consistent, and this etymology speaks to the unifying progressive principle: active egalitarianism.
In America, two major principles have emerged as touchstones for egalitarian progress. The first comes from the 18th to the late 19th century, the equality of opportunity. That principle accepted inequality of income, health, and other circumstances as natural but maintained that people of low-status could rise to the top through industry. Abraham Lincoln, before his first inauguration, said that that the principle that held the union together was the promise that “all should have an equal chance.” This principle was appropriate for a nation that was largely agrarian and in which an empire of land lay unexploited, cheaply available on the frontier.
Three constraints ultimately limited the upward mobility of Americans. First, owing to new technology and larger markets, the scale of American industry expanded dramatically with the rise of the steel, railroad, and oil industries and new workers could no longer expect to one day own their businesses. Second, increased immigration from Europe lowered wages in cities. Most importantly, the Frontier was declared closed in the 1890s and with that went the once infinite source of possibility--the West, which had enabled opportunity for centuries. Equality of opportunity was a false promise shortly after the civil war.
These constraints gave raise to a new principle whose touchstone was equality of condition rather than of opportunity: this was the birth of the progressive movement, in my mind. Greater equality of conditions was largely to be achieved through government policy (restricting child labor, limiting the work week, restricting immigration, and strengthening unions). Income was also to be transferred from the rich to the poor through progressive income tax. Abraham Lincoln signed Revenue Act of 1862, which instituted the first federal income tax--before being overturned by the Supreme Court some thirty years later. William Jennings Bryan and his Populist Party fought for a more progressive income tax in the late 1890s, Theodore Roosevelt founded the Progressive Party (Bull Moose Party) in the election of 1912, and in 1913 Congress passed the 16th Amendment, which enabled a non-apportioned federal income tax.
The progressive movement’s legacy is not simply an emphasis on redistribution; it was the realization that the old ethos did not work for the modern economy. So to me, what distinguishes a progressive is not the lonely and simple goal of “more redistribution” to the extent of the modern welfare state (that’s not Theodore Roosevelt’s Progressivism), nor even a strict defense of equality of condition. Rather unifying principle is an adapted notion of egalitarianism that recognizes that the promise of equality of opportunity will no longer be fulfilled by an abundance of wealth and capital beyond the frontier. Rather, the state must actively enable equality of opportunities.
More recently, the older, do-it-yourself version of egalitarianism has come back to favor, enabled by a marriage of convenience between a wealthy elite who are not interested in egalitarianism, and a coalition of evangelicals whose faith emphasizes personal responsibility. All I can say for the former group is that their politics are consistent with their interests. As for latter, who have become the most powerful political coalition in American politics, their emphasis on personal responsibility is based on the insane and willfully-dishonest notion that equal opportunity is possible without government support. The most important plank of progressive ideology rejects this laissez-faire approach to egalitarianism.