In the second sentence, "get across" is a set expression (phrasal verb) meaning to communicate successfully, or successfully make people hear and understand what you are saying. The phrasal verb "get across" does not take a separate object for "across", so "get" and "across" in "get my message across the country" have to be interpreted differently, more as separate words than as a set idiomatic combination. The meaning is something like "broadcast/spread my message across the country".
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