CREATE TABLE table_18161 ("District" TEXT, "Incumbent" TEXT, "Party" TEXT, "First elected" TEXT, "Results" TEXT, "Candidates" TEXT)

Incumbent Tim Holden belonged to what party?
SELECT "Party" FROM table_18161 WHERE "Incumbent" = 'Tim Holden'
SELECT "Party" FROM "table_18161" WHERE "Incumbent" = 'Tim Holden'