user: Question: In what year did the Democratic incumbent that was Re-elected first take office?
Columns: ['District', 'Incumbent', 'Party', 'First elected', 'Results']
Types: ['text', 'text', 'text', 'real', 'text']
Sample row: ['Iowa 1', 'Jim Nussle', 'Republican', 1990.0, 'Retired to run for Governor Democratic gain']
SQL:

assistant: SELECT "First elected" FROM "Table" WHERE "Results" = 'Re-elected' AND "Party" = 'Democratic';