PL/pgSQL function syntax question?

i get the following error when i try and create the following function:

Basically, i am trying to have two different dates compared and only the
most recent returned to me. This seems pretty straight forward, what I am
doing wrong here?

create or replace function fcn_pick_date(v_dt date)
returns date as $$
DECLARE
v_dt date;
BEGIN
for v_record in select last_periodic, last_boot
from mediaportal
loop
if v_dt >= v_record.last_boot then
v_dt := v_record.last_periodic;
else
v_dt := v_record.last_boot;
end if;
end loop;
return (v_dt);
END;
$$ LANGUAGE 'plpgsql';

ERROR: loop variable of loop over rows must be record or row variable at or
near "loop" at character 195

********** Error **********

ERROR: loop variable of loop over rows must be record or row variable at or
near "loop"
SQL state: 42601
Character: 195