CREATE TABLE table_18561 ("District" TEXT, "Incumbent" TEXT, "Party" TEXT, "First elected" FLOAT, "Result" TEXT, "Candidates" TEXT)

When the result is retired to run for U.S. Senate Democratic Hold, who is the candidate?
SELECT "Candidates" FROM table_18561 WHERE "Result" = 'Retired to run for U.S. Senate Democratic hold'
SELECT "Candidates" FROM "table_18561" WHERE "Result" = 'Retired to run for U.S. Senate Democratic hold'