Sydney - Liberal Party lawmakers arrived Tuesday at Canberra's Parliament House for a leadership ballot that pits struggling incumbent Brendan Nelson against his treasury spokesman Malcolm Turnbull.

Nelson, 51, declared the position vacant in the hope that in a new vote he could improve on the 45-42 winning margin he achieved nine months ago.

Nelson, a former general practitioner, is woefully behind Prime Minister Kevin Rudd in the opinion polls. His party is further behind Rudd's Labor than it was when losing the general election in November under John Howard.

Rudd has enjoyed record support since driving Howard from office and out of Parliament.

"It's a very high risk strategy," said Howard biographer Peter van Onselen, predicting that a narrow win for the dour Nelson would only perpetuate instability within the Liberals.

Turnbull, 54, an ebullient former barrister and merchant banker who entered Parliament in 2004, goes into the spill free from the stigma of having called it. dpa