The reason why Father Coughlin's endorsement of Wendell Willkie damaged the latter's chances was that Father Coughlin supported Hitler and Mussolini.
At the time that Wendell Willkie was running for President in 1940, the second World War was raging in Europe and Hitler's horrible policies were being condemned in the U.S.
Father Coughlin had once voiced support for the Fascist policies of Hitler and Mussolini so when he endorse Wendell Willkie, it led to many thinking that Mr. Willkie was also a supporter of Fascism which led to his loss.
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