Some, like Tim Fischer and Brendan Nelson, have taken the government's shilling in the form of overseas posts. Others, like Alexander Downer, have become lobbyists and public commentators in a way that has submerged somewhat their partisan and factional allegiances. Peter Costello has done a bit of both. Even John Howard himself seems to be increasingly a little bit above the fray, or only engaged in political battles on a part-time basis.

But Minchin is still in the trenches, because he has always been a true believer. Tonight he is appearing in a special ABC television documentary called I Can Change Your Mind About Climate, followed by Q&A, in his role as a leading climate sceptic. He is an unusual choice as it pits an experienced older man against a younger woman in a predictable way. Last weekend he was writing once again to a major national paper to defend the record of the Howard government, attacking some innocuous reflections by one of his regular Liberal targets, Malcolm Turnbull.

Minchin entered the Senate in 1993 after 15 years working for the Liberal Party federally and in South Australia. His early years in the parliament were an extension of his role as a party machine man, because he was successively parliamentary secretary to the leader of the opposition, parliamentary secretary to the prime minister, minister assisting the PM and special minister of state. All up this made almost 20 years as a machine man and minister for politics. He is one of those politicians who had virtually no career outside of politics, his great love.

There have been others like him on both sides of politics, including Sir John Carrick (Liberal) and Mick Young (Labor), who have crossed over from state party secretary to minister for politics. But it is difficult to think of anyone else with such unwavering beliefs who was immersed in party organisational politics for so long.

He was a faction leader in the South Australian Liberals and nationally, earning a reputation as a take-no-prisoners factional warrior. In that role he recruited acolytes like Senator Cory Bernardi, and fought the small-l liberals inside his party, especially his main adversary Christopher Pyne, at every turn, including at each leadership contest involving Turnbull.

An early task was to mastermind for Howard the No case at the 1999 republic referendum. He was formidable in that role and did not ever let it drop afterwards, maintaining his active connections with monarchist organisations.

During the 1990s he was almost a lone voice advocating voluntary voting. In an early Senate minority report he declared that he was not convinced that passive smoking was dangerous. To some these positions were just quirky, even a little crazy, while to others it was a sign that he wasn't afraid to challenge conventional wisdom.

He became minister for industry, science and resources in 1998 and later ended up as Australia's longest serving finance minister. Yet he stood down from politics as a relatively young man, not yet 60, and left the Senate in the middle of last year. His son had suffered a serious injury while training at the Australian Defence Force Academy and this drew Minchin back to his family in a life-changing way.

Without speculating on his decision, because he had been in Parliament almost 20 years and had the right to go, he certainly had a lot of politics still left in him and this has shown itself since his departure from the formal arena. He is still very much a political animal and continues to seek a national voice.

Some of his interventions are predictable. He joined those who lambasted the Minister for Defence, Stephen Smith, over his standing down of the ADFA Commandant over the Skype sex scandal, for instance. He is a carbon-tax critic too, of course, but with a fundamentalist fury that may actually unnerve middle-of-the-road Liberal opponents of the tax. He's not just playing politics, because he deeply believes what he is saying.

But other interventions are not predictable and do not just follow the party line. He stood up for the government when the Coalition opposition criticised the Gillard government's $500 million support for the car industry. Perhaps that is the South Australian in him. But he also took an independent position over the government's appointment of David Gonski as chair of the Future Fund. In his view Peter Costello should never have been considered as a contender given his party-political background.

I can't say what drives Minchin. I've only met him once and that was in the mid-1990s when, over dinner after a Constitutional Centenary Foundation public meeting, I glimpsed his fierce and unbending opposition to a republic. He comes across as an extremely conservative, hard-line machine man and for that reason we have very little in common.

On some things he strikes me as way beyond the mainstream.

But despite that he does appear to be an independent thinker on some subjects.

He also has staying power and when he champions causes he commits to them for the long haul in a way that few others on either side of politics do.

You can understand why some people greatly value his support for this reason and why others fear him because of it.

John Warhurst is an emeritus professor of political science at the Australian National University.

John.Warhurst@anu.edu.au