A day after Labor locked in the support of Tasmanian independent Andrew Wilkie, taking it to within two seats of forming a government, Coalition MPs made a desperate bid to secure the backing of the remaining three hold-outs.

Opposition MPs yesterday ramped up the pressure on independents Bob Katter, Rob Oakeshott and Tony Windsor to join the Coalition, warning that a Labor government would risk a major shift toward left-wing politics.

The three men said they had yet to decide which party to support and hoped to make that decision by early next week.

With Labor having secured 74 seats, the Coalition, which has 73 seats, needs the support of all three to be able to form government.

In a sign they feared the momentum had swung in Labor's favour, senior Coalition figures, including former prime minister John Howard, yesterday took aim at the Greens' move to support Labor's bid to form minority government.

Both shadow treasurer Joe Hockey and opposition finance spokesman Andrew Robb warned the alliance between Labor and the Greens - which extends only to matters of supply and no-confidence motions - would deliver the most left-wing government in Australia's history.

"It is inconceivable to me that country independents would choose to go with what would be the most centre-left government in Australian history, in a Green-Labor alliance," Mr Hockey said.

It was a clear line of attack, with Mr Robb making an almost identical comment.

Mr Howard, speaking at a fundraiser for children with leukaemia in Brisbane, made a similar appeal to the independents.

"The alliance that was sealed between the Australian Labor Party and the Australian Greens does represent a significant signal that the politics of a future Labor government would be to the left of what has been the politics of the Labor government of recent years," he said.

He said the next Labor government would be further to the left than the governments of Bob Hawke and Paul Keating between 1983 and 1996, as well as that of the recent governments of Kevin Rudd and Julia Gillard.