Question: Say a group of people vote for a leader in an election but the leader then goes on to "*speak for the people*" but actually he's speaking for himself and his cronies. I'd like it to be clearly in a derogatory sense.

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> "Mr ..... claimed he was speaking on behalf of the people who voted for him but he was actually **[stealing]** their voice"
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What word would best fit here?

Answer: 
The most suitable word for my context was "Highjack" bringing strong, explicit implication of stealing for their own purposes. The word was contributed in a comment that was subsequently removed. Thanks to that unknown contributor!
You could use the term *co-opt*:

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> **co-opt** *v. tr.*  
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>  3. To take or assume for one's own use; appropriate: *co-opted the criticism by embracing it.*
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[From TFDO](http://www.thefreedictionary.com/co-opt)