Prime Minister Scott Morrison has warned Wentworth by-election voters of the risk of a hung parliament and economic uncertainty if Liberals don't retain the seat.
He admitted the Sydney seat, which was formerly held by Malcolm Turnbull, was a three-horse race between his own party, Labor and independent Kerryn Phelps.
Mr Morrison warned voters a Liberal loss could mean a hung parliament which would create "unnecessary uncertainty in our economy and the stability of our government".
"If you don't vote for the Liberal candidate at this election ... you risk a hung parliament," Mr Morrison told reporters in Murray Bridge, South Australia on Saturday.
While singing the praises of Liberal candidate Dave Sharma, the prime minister urged Liberals who were considering voting for Dr Phelps to "think about that carefully".
"(Dr Phelps) is clearly running strongly second and if she continues in that place, she can still be a long way behind on primaries and then she can come over the top and win that by-election," he said.
Mr Sharma agreed with the prime minister: "It's simple maths - if anyone other than the Liberal candidate is elected then we will have a hung Parliament, meaning greater uncertainty for our economy and our country."
The Liberal candidate also responded to questions over his stance on climate change, which has been a hotly debated topic during by-election campaigning.
Mr Sharma said addressing climate change and keeping international commitments was important.
"I have been on the record for some time. I accept the science of climate change," he said.
Federal Opposition Leader Bill Shorten believes the October 20 by-election is an opportunity for fed up voters to send a message to the government.
The Labor leader says voters are "stunned" with the Morrison government's inaction on climate change following the release of a major United Nations report earlier in the week.
"If you are not happy with the way this government has been running things for the last two-and-a-half and indeed five years send them a message - 'not this time'," he told reporters in Sydney on Saturday.
Dr Phelps also hit out at the "revolving door of leadership" caused by the major parties in government and criticised both Labor's and the coalition's climate change policies.
"The Morrison government is hopelessly divided on climate science and what to do about it, while Labor's plan doesn't go far enough," she told AAP in a statement on Saturday.
Labor's candidate for Wentworth Tim Murray admitted it would be a "miracle" for the Liberals to lose given their margin but said voters are sick of the party taking them for granted.
Former Prime Minister Malcolm Turnbull resigned from his seat after losing the Liberal leadership at the end of August, sparking the by-election.