During the 1930s black voters overwhelmingly switched from the republican party to the democratic party because

Black Americans benefited from some New Deal economic policies.

Explanation:

The New Deal economic policy was formed by President Franklin Delano Roosevelt in the 1930s. This policy was formed to combat the economic crises during the Great Depression.

According to the New Deal, opportunities in jobs and assistant programs were given and governmental reforms were formed to come out of the Great Depression. Roosevelt was a Democrat and his New Deal provided relief, recovery, and reformation from the Great Depression.

The black voters during this time, realigned themselves to Democrats shifting from the Republicans. This shift occurred because of the New Deal that provided them with job opportunities and relief from economic crises.