carr threat to gillard, page-4

"Who was our last senator who became PM?"

If your memory goes back that far, Liberal Senator John Gorton became the prime minister of Australia on 10/1/1968 while still a senator. He resigned his senate seat on 1/2/1968 to contest a House of representatives seat, the seat of Higgins which he won handsomely.

By convention, the prime minister has to come from the House of Representatives.