Question

why did senator huey long not mount a presidential bid against fdr?
long was incidentally assassinated before he had resolved to run for the presidency.
long was forced into retirement by attack ads placed by his powerful opponents.
since long and fdr were both democrats, the party talked him into backing down.

Explanation:

Huey Long was assassinated in 1935. At that time, he was considering a presidential bid against FDR. His assassination prevented him from actually mounting that bid. There is no historical evidence to support the other two options. Attack ads forcing him into retirement is not in line with historical facts. And the idea of the party talking him down due to both being Democrats is also not a valid historical reason for his not running as he was already planning to potentially challenge FDR within the party (if he had lived).

Answer:

Long was incidentally assassinated before he had resolved to run for the presidency.