Sterle's involvement with the union prompted him to join the
Australian Labor Party in 1991, and in 1999, he was elected as a delegate to the party's state conference. He served as the ALP's transport policy convener in 2000, and was a delegate to the party's national conference in 2002 and 2004. He subsequently decided to make a bid to enter parliament, and in the lead-up to the
2004 federal election, challenged the preselection of veteran senator and former cabinet minister
Peter Cook. Cook was determined to remain in parliament, but withdrew from the ballot of their shared Centre Faction when it became clear that Sterle had achieved enough support to win. As a result, Sterle gained the second position on the party's Senate ticket and was easily elected. ==Senate==