user: Question: What was the smallest lead margin when the source of polls came from Strategic Vision and Democrat Carl Levin had 57%?
Columns: ['Poll Source', 'Dates administered', 'Democrat: Carl Levin', 'Republican: Jack Hoogendyk', 'Lead Margin']
Types: ['text', 'text', 'text', 'text', 'real']
Sample row: ['Rasmussen Reports', 'October 8, 2008', '61%', '36%', 25.0]
SQL:

assistant: SELECT MIN("Lead Margin") FROM "Table" WHERE "Poll Source" = 'Strategic Vision' AND "Democrat: Carl Levin" = '57%';