JFK's Catholic speech (along with his father's money) clearly assuaged the concerns of enough protestants to allow him to win the nomination and the presidency. In that regard, I think it's reasonable to conclude the JFK speech was a success.
While Mitt Romney's Mormon speech was well-received and widely praised, it was not so good as to have helped him win the nomination. Clearly, there were many factors involved in how the race played out -- including Huckabee winning Iowa and taking votes away from him in South Carolina. Still, exit polls would confirm that on election day, some voters were still influenced by anti-Mormon bias, meaning Romneys' speech -- though well-delivered -- did not persuade the folks who needed persuading.
It is important to note that a speech's efficacy should be based not on whether or not it is immediately praised by insiders (this may include writers, pundits, and even bloggers) -- but on whether or not it achieves the long-term objective of persuading actual voters. In this regard, I think Obama's speech fell short.
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