Final answer:
During Taft's presidency, the Progressive Republicans were initially hopeful but ultimately found themselves divided and marginalized. Taft's policies, such as supporting the Payne-Aldrich Act and the U.S. Chamber of Commerce, alienated many Progressives, leading to a split in the Republican Party and a weakened Progressive political organization post-election.
Step-by-step explanation:
Under President William Howard Taft's administration, Progressive Republicans experienced a mix of outcomes. While Taft began his presidency with surprising vigor in pursing antitrust legislation, even more so than his predecessor Theodore Roosevelt, he did not consistently ally with the Progressive ideals. Over time, Progressive Republicans found themselves increasingly at odds with Taft, particularly over issues like the Payne-Aldrich Act which raised tariff rates and the creation of the U.S. Chamber of Commerce, seen as countering labor's influence.
The Progressive Republicans were further alienated when Taft supported conservative candidates in the 1910 House and Senate elections. All of these actions culminated in a division within the party, with some Progressives continuing to support Taft while others vehemently opposed him, leading to Roosevelt's run as a third-party candidate in 1912. After the elections, many who had backed the Progressive Party eventually returned to the Republican Party, and the Progressive political organization diminished swiftly, signaling that Progressive Republicans were divided and marginalized by the end of Taft's administration.