CREATE TABLE table_18504 ("District" TEXT, "Incumbent" TEXT, "Party" TEXT, "First elected" FLOAT, "Result" TEXT, "Candidates" TEXT)

What district does Harlan Hagen represent?
SELECT "District" FROM table_18504 WHERE "Incumbent" = 'Harlan Hagen'
SELECT "District" FROM "table_18504" WHERE "Incumbent" = 'Harlan Hagen'