CREATE TABLE table_21793 ("Poll Source" TEXT, "Dates administered" TEXT, "Democrat: Jay Nixon" TEXT, "Republican: Kenny Hulshof" TEXT, "Lead Margin" TEXT)

Who's the poll source that claimed the Lead Margin was 17 and the Democrat: Jay Nixon had 54% of the votes?
SELECT "Poll Source" FROM table_21793 WHERE "Democrat: Jay Nixon" = '54%' AND "Lead Margin" = '17'
SELECT "Poll Source" FROM "table_21793" WHERE "Democrat: Jay Nixon" = '54%' AND "Lead Margin" = '17'