query: SELECT ?item ?party ?district ?start ?end ?cause WHERE {
    ?item p:P39 ?statement.
    ?statement ps:P39 wd:Q21030356;
      pq:P2937 wd:Q20878969.
    OPTIONAL { ?statement pq:P580 ?start. }
    OPTIONAL { ?statement pq:P582 ?end. }
    OPTIONAL { ?statement pq:P1534 ?cause. }
  
    OPTIONAL { ?statement pq:P768 ?district. }
    OPTIONAL { ?statement pq:P4100 ?party. } 
  }