"First of all, Sen. Gardner is clearly representing his constituents by voting how those who elected him would prefer." 
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You are conflating "constitutients" with "supporters". His constituents are ALL the voters of Colorado (including those who voted against him and those who didn't vote in that election). 
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" Those on the right would obviously find, say, Sen. Michael Bennet to be "radical left" and that Sen. Gardner fits right into their view."
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Actually, a lot of those on the right are on record as finding Bennet to be "moderate left" rather than radical.