What was the group of conservative businessmen established in 1934 that vehemently attacked the administration for what they considered its attack on property rights, the growing welfare state, and decline of personal liberty?

American Liberty League.

Explanation:

American Liberty League was established on 15 August 1934 by wealthy businessman and political leaders who were anti-Roosevelt democrats and conservatives. This American protest organization was formed to defeat Roosevelt and restrict the trade union movement. They claim to fight the socialism and dictatorship of Roosevelt's administration. However, the League eventually died with the re-election of Roosevelt for the second term.