What are the dates where Hulshof had 37% and Nixon had 48%?
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" real
);
SELECT "dates_administered" FROM "opinion_polling_for_the_united_states_gu" WHERE "republican_kenny_hulshof"='37%' AND "democrat_jay_nixon"='48%';