Kevin Rudd faces an uphill battle at home and abroad to be the next United Nations Secretary-General, with conservative MPs pushing the Turnbull government not to endorse him and overseas observers saying he falls well short of the necessary international backing.

Mr Rudd has yet to announce his candidacy but Foreign Minister Julie Bishop has confirmed he is lobbying for the diplomatic post. Some observers, who spoke to Fairfax Media on condition on anonymity, said Mr Rudd should not be written off but the weight of opinion saw his candidacy as facing significant obstacles.

Liberal MPs Tony Pasin and James Paterson this week became the latest to speak out against Mr Rudd's candidacy and urge the Turnbull government not to give the former prime minister its imprimatur.

"This guy did his best to ruin our country," Mr Pasin said. "That we would think about imposing him on the rest of the world beggars belief."