CREATE TABLE table_36746 ("District" TEXT, "Incumbent" TEXT, "Party" TEXT, "First elected" TEXT, "Result" TEXT)

Who was the incumbent that retired to run for the senate democratic hold?
SELECT "Incumbent" FROM table_36746 WHERE "Result" = 'retired to run for the senate democratic hold'
SELECT "Incumbent" FROM "table_36746" WHERE "Result" = 'retired to run for the senate democratic hold'