What led to the formation of the Progressive party in U.S. politics?

The
National Progressive Party, also called the Bull Moose Party, was the result of a split in the
Republican Party in the 1912 election between Theodore Roosevelt and Howard Taft, Roosevelt's
hand-picked successor.  Roosevelt left office in 1909 under the impression that Taft would
continue the progressive line blazed by Roosevelt, "to dissolve the unholy alliance between
corrupt business and corrupt politics," as the Progressive Party platform put it.  When
Taft did not continue this line, Roosevelt challenged Taft's bid for reelection.  This split in
the Republican Party made it easier for Woodrow Wilson to win the presidency in
1912.

The Progressive Party dissolved within two years following the election,
but Roosevelt's view of government's direct care for its citizens would affect later policies,
specifically the New Deal of Franklin Delano Roosevelt, TR's distant
cousin.