Liberal senator Cory Bernardi has insisted cabinet reject Kevin Rudd’s bid to become secretary-general of the United Nations, saying any nomination would validate his “disastrous” reign as prime minister.

The Coalition is expected to support Mr Rudd’s nomination  for the position without officially campaigning for his appointment, in a nuanced position designed to appease conservatives within government ranks.

Senator Bernardi today rejected that compromise, saying the Coalition could not “in good conscience” support Mr Rudd knowing that he was not “fit” for the top role in global diplomacy.

“Some claim that supporting his nomination is no endorsement for the role but it would be hard to see it as anything other than a validation of his disastrous national stewardship,” he wrote in his weekly newsletter to supporters.

“While Rudd is unlikely to win the position, how in good conscience, having lived through the Rudd leadership and destabilisation, can any of us suggest he is fit for the role he seeks?”

Senator Bernardi rejected the “parochial” argument that Australia must support any Australian for any position, fearing it would damage our international reputation.

He also rejected suggestions Mr Rudd could be no better than other “unworthies” selected to fill UN posts, saying Australia should not “abandon reason and suspend critical judgment simply because others do”.

“The world is in a difficult state. We face almost unprecedented levels of global debt, terrorism, social dislocation and a crisis of confidence in government. Whatever its failings (and there are many), the UN attempts to resolve some of those problems and needs the best possible leadership,” he wrote.

“Why would we make their job any more difficult by throwing Kevin Rudd into that mix?”

However, Mr Rudd has received the backing of former Liberal Leader Brendan Nelson in his mission to become UN Secretary General with his old political rival today declaring Mr Rudd was made for the job.

Dr Nelson – now the Director of the Australian War Memorial — said Mr Rudd was “stunningly brilliant” in his grasp of foreign affairs and urged people to overcome their prejudices or personal dislike for the man and back in an Australian UN Secretary General.

Speaking at the National Press Club in Canberra, Dr Nelson said Mr Rudd’s elevation to the role as Ban Ki-moon’s successor would make him proud as an Australian and argued he would exceed people’s expectations.

Reflecting on his time as Opposition Leader, Dr Nelson said he often quipped that the former Labor Prime Minister had a plan for the world while lacking one for Australia.

“I could spend quite a lot of time having a discussion with you about Mr Rudd’s failings, but I think this is an occasion where we need to be at our best as Australians,” Dr Nelson said. “This is a team Australia event. Some people are tailor made for some jobs… And I think Kevin Rudd’s made for this job.”

“I can tell you that as Australia’s ambassador to NATO, on one of his visits to Brussels as Foreign Minister, I took Kevin Rudd to the North Atlantic Council. He spoke to the heads of the military of the 28 NATO countries, the permanent representatives and senior political figures of those member countries… He wasn’t good. He was stunningly brilliant.”

Dr Nelson said Mr Rudd had a natural instinct and skill for diplomacy that would be the “envy of anybody that’s ever worked in the diplomatic space.”

Opposition deputy leader Tanya Plibersek yesterday insisted Mr Rudd had served Australia “with distinction” and challenged Malcolm Turnbull to heed his own calls for bipartisanship.

“He is obviously a very strong candidate and the idea that the Australian government for party political reasons would not back his candidacy is incredibly small-minded,” Ms Plibersek said.

“We have a Prime Minister who is out there saying that there should be more bipartisanship in public life — well here is an opportunity to show that bipartisanship.”