The election of Woodrow Wilson as president in 1912 represented the high water mark of progressive politics. Progressivism had been in the ascendant for a number of years, with the spirit of this great social and political movement influencing the policies of both parties.
However, whereas Progressivism could more easily be absorbed into the policy program of the Democratic Party, a party increasingly representing the interests of working people, it found a more hostile reception among most members of the Republican Party, who were deeply suspicious of what they perceived as its radical leanings.
The result was a huge split in the GOP, which led to former president Roosevelt running as a Progressive against the Republican Party, whose candidate was the sitting president, William Howard Taft. In the ensuing contest, Wilson was able to win the presidency, largely due to the split among his Republican opponents.
The election of 1912, then, can be seen as a strengthening of the two-party system in the United States. From then on, the message was clear: meaningful change of any description could only come from within the existing parties, not in opposition to them. Although third-party candidates have run on numerous occasions since then, none of them have been able to break the stranglehold that the Democratic and Republican parties have had on the American political system.