Who's the poll source that claimed the Lead Margin was 17 and the Democrat: Jay Nixon had 54% of the votes?
CREATE TABLE "opinion_polling_for_the_united_states_gu" (
  "poll_source" text,
  "dates_administered" text,
  "democrat_jay_nixon" text,
  "republican_kenny_hulshof" text,
  "lead_margin" text
);
SELECT "poll_source" FROM "opinion_polling_for_the_united_states_gu" WHERE "democrat_jay_nixon"='54%' AND "lead_margin"='17';