Who was Churchill referring to when he said, "The most powerful democracy have declared... that they will devote their overwhelming industrial and financial strength"?

The United States of America

Explanation:

Winston Churchill was referring to The United States, thanking them for enacting the Lease and Lend Bill, which, prior to American involvement in World War II, allowed the U.S government to send material aid such as weapons and food to the United Kingdom and any other nation deemed vital to "the defense of the United States" with America still remaining officially neutral, at this time.  

a. favoring native born americans over immigrants.