CREATE TABLE table_18644 ("District" CLOB, "Incumbent" CLOB, "Party" CLOB, "First elected" FLOAT, "Result" CLOB, "Candidates" CLOB)

Who are all the candidates when Sam Rayburn was incumbent?
SELECT "Candidates" FROM table_18644 WHERE "Incumbent" = 'Sam Rayburn'
SELECT "Candidates" FROM "table_18644" WHERE "Incumbent" = 'Sam Rayburn'