user: Question: If the registered voters is 66.8%, what is the minimum population?
Columns: ['City', 'Population', 'Registered voters', 'Democratic', 'Republican', 'D–R spread', 'Other', 'No party preference']
Types: ['text', 'real', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Campbell', 39108.0, '53.4%', '45.3%', '23.8%', '+21.5%', '7.3%', '26.2%']
SQL:

assistant: SELECT MIN("Population") FROM "Table" WHERE "Registered voters" = '66.8%';