CREATE TABLE table_18378 ("District" TEXT, "Incumbent" TEXT, "Party" TEXT, "First elected" FLOAT, "Result" TEXT, "Candidates" TEXT)

When was the first elected when the party was republican and the candidate were robert h. michel (r) 66.1% rosa lee fox (d) 33.9%?
SELECT MIN("First elected") FROM table_18378 WHERE "Party" = 'Republican' AND "Candidates" = 'Robert H. Michel (R) 66.1% Rosa Lee Fox (D) 33.9%'
SELECT MIN("First elected") FROM "table_18378" WHERE "Candidates" = 'Robert H. Michel (R) 66.1% Rosa Lee Fox (D) 33.9%' AND "Party" = 'Republican'