What was the smallest lead margin when the source of polls came from Strategic Vision and Democrat Carl Levin had 57%?
CREATE TABLE "opinion_polling_for_the_united_states_se" (
  "poll_source" text,
  "dates_administered" text,
  "democrat_carl_levin" text,
  "republican_jack_hoogendyk" text,
  "lead_margin" real
);
SELECT MIN("lead_margin") FROM "opinion_polling_for_the_united_states_se" WHERE "poll_source"='Strategic Vision' AND "democrat_carl_levin"='57%';