Liberal leader

In October 2005, he moved to challenge then Liberal leader Rob Kerin, but later withdrew his challenge.^[11] On 11 April 2007, Hamilton-Smith formally challenged then Liberal leader Iain Evans, and was successful with 13 votes to 10 for the Liberal leadership.^[12] On 19 April 2007, he announced a re-shuffle of the opposition front bench.^[13]

Under his leadership, polling by Newspoll saw the Liberals go from 29 to 40 per cent on the primary vote, and from 39 to 50 per cent on the two party preferred vote. The Preferred Premier rating saw Hamilton-Smith start on 21 per cent, seven points higher than his predecessor, to a high of 30 per cent, with Rann falling from a high of 64, to 48 per cent. However, Newspoll saw Labor back in a winning position on 54 to 46 in late 2008, and then 56 to 44 in early 2009 along with a widening gap in the Preferred Premier rating. During the 50-50 polling, The Sunday Mail polling suggested that whilst there had been large swings away from the government in country areas, support was holding relatively firm at 2006 election levels in the metropolitan areas.^[14] However, this did not play out at the 2009 Frome state by-election sparked by the parliamentary resignation of former Premier Rob Kerin, which saw a rare two-party swing from the opposition to the government, and resulted in independent Geoff Brock taking the seat from the Liberals on preferences.^[15][16][17]

2009 leadership spill

Hamilton-Smith accused Labor of accepting split donations from the Church of Scientology based on information sent to the Liberal Opposition that was subsequently found to have been forged.^[18] This controversy coupled with the Frome by-election and continued poor polling, saw Liberal MPs openly talk of a leadership change, with a high chance of a leadership spill likely, prior to the 2010 state election.^{[19][20][21][22][23][24][25][26]} Two days later, Hamilton-Smith announced a spill of the leadership and deputy leadership, with a ballot taking place on Saturday 4 July 2009.^[27][28] Williams, Isobel Redmond, and Iain Evans ruled out contesting the ballot, with Hamilton-Smith and his moderate deputy Vickie Chapman the only contenders.^[29] Hamilton-Smith defeated Chapman in the leadership spill, 11 votes to 10, with one MP abstaining. At first, Hamilton-Smith immediately announced he would stand down, which would have delivered the leadership to Chapman. Hamilton-Smith announced a second leadership ballot to be held on Wednesday 8 July. On Monday 6 July, Hamilton-Smith confirmed he would not be running for the leadership. Contenders for the leadership were Chapman, Redmond, and Williams. Hamilton-Smith and his supporters backed Redmond.^[30][31] Redmond won the leadership spill on 8 July 2009, 13 votes to 9 against Chapman.^[32]

Deputy Leader

On 30 March 2010, Hamilton-Smith was elected deputy leader of SA Liberals to replace Steven Griffiths in a party-room vote, defeating Iain Evans 10 votes to 8. He once again defeated Evans in a leadership position in a rematch between the two former leaders.

2012 leadership spill

Hamilton-Smith nominated for the position of South Australian Liberal parliamentary leader, with Steven Marshall as deputy leader after Hamilton-Smith declared a leadership spill against Isobel Redmond and Mitch Williams.^[33] A party room ballot occurred on 23 October 2012, Redmond retained the leadership by one vote, however Marshall was elected to the deputy leadership.^[34]

Redmond resigned on 31 January 2013. Hamilton-Smith chose to support Steven Marshall who was elected leader unopposed. Hamilton-Smith was appointed Shadow Minister for Economic and Regional Development, Mineral Resources and Energy, Manufacturing, Industry and Trade, and Defence Industries in the subsequent reshuffle.^[35]

Independent Liberal in a Labor cabinet

The 2014 election resulted in a hung parliament with 23 Labor seats, 22 Liberal seats, and two independents. The balance of power was held by crossbench independents Geoff Brock and Bob Such. Such did not indicate who he would support in a minority government before he was diagnosed and hospitalised with a brain tumour and took medical leave. With 24 seats required to govern, Brock backed Labor. He accepted the cabinet positions of Minister for Regional Development and Minister for State and Government Local Relations. Brock agreed to support the Labor government on confidence and supply while retaining the right to otherwise vote on conscience.^[36]

On 27 May 2014, more than two months after the election, in a media conference with South Australian Labor Premier Jay Weatherill, Hamilton-Smith announced his decision to resign from the South Australian Liberal Party to become an "Independent Liberal" MP, and to join the Labor cabinet as the Minister for Trade, Defence Industries and Veterans' Affairs. Though his vote was not crucial to the government, he agreed to support the Labor minority government on confidence and supply while retaining the right to otherwise vote on conscience, stating that South Australian business needed a stable parliament.^[37][38]

An Advertiser poll of 350 Waite voters was conducted a few days after Hamilton-Smith's announcement. On the question of "should there be a by-election in Waite", 43 percent said no, 41 percent said yes. On the question of "do you feel betrayed by his decision", 46 percent said no, 42 percent said yes.^[39]

Labor unexpectedly won the 2014 Fisher by-election by five votes, following a 7.3 percent swing, which gave them majority government. Despite that, the Weatherill government kept Brock and Hamilton-Smith in cabinet, giving it a 26 to 21 parliamentary majority.^[40]

Hamilton-Smith was appointed to the ministerial portfolio of small business in 2016. In September 2017, he was appointed minister with responsibilities for space industries and health industries. Major achievements over four years as a minister included helping to retain $90 billion worth of submarine and shipbuilding work in SA,^[41] establishing Investment Attraction South Australia,^[42] construction of the Centenary of ANZAC Memorial Walk along Kintor Avenue,^[43] hosting the International Space and Astronautical Congress in Adelaide as Australia's first space industries minister,^[44] and improving communication of small business through the establishment of a small business roundtable.

While serving as Leader of the Liberal Opposition, Hamilton-Smith led the debate to move football from Westlakes to the city, resulting in the redevelopment of Adelaide Oval.^[45]

On 6 January 2018, Hamilton-Smith announced that he would retire at the 2018 state election and resigned from the Weatherill Ministry on 17 January 2018.^[46]