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