" implies that you have previously considered the matter and made up your mind to vote in that way. " merely implies a willingness to vote that way, without such a strongly-held or long-held plan or intention to do so. For example, if a canvasser was trying to persuade you to vote for Mr Jones, you might say "Sorry, I'll be voting Smith" ("or "I'm voting Smith"), but "OK then, I'll vote Jones".
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