Tony Abbott has needled the former prime minister Malcolm Turnbull to publicly endorse the Liberal candidate for Wentworth Dave Sharma as the critical byelection contest enters its closing week.

Abbott used his regular spot on 2GB to urge Turnbull to spruik Sharma on social media, contending he owed that to the Liberal party and his former constituents in Sydney’s eastern suburbs.

Kerryn Phelps urges Wentworth voters to use byelection to protest 'inhumane' refugee policies Read more

“I know he doesn’t want to get too involved with Australian politics, I understand that, I know he is probably enjoying a bit of R&R with Lucy in New York, but I reckon he owes it to the party and the people of Wentworth to give Dave Sharma a solid, clear personal endorsement this week in particular,” Abbott said on Monday morning.

Abbott declined to criticise Turnbull’s son Alex (who has been active on social media throughout the contest blasting the Liberal party’s inaction on climate change), saying “let the son speak for himself”, but he said Turnbull senior could show “a bit of explicit support for Dave Sharma”.

Play Video 1:10 ‘No way coal can compete anymore’: Malcolm Turnbull's son wants voters to dump Liberals – video

Turnbull has been overseas since the Liberal party took the decision to oust him from the prime ministership – the event that has triggered the byelection contest that could see the Morrison government lose its one-seat majority in the House of Representatives.

Turnbull made it clear when the conservative faction moved against him in late September that he would not remain in the parliament if he lost the party leadership. The warning did not stop the move against him, and Turnbull resigned after Scott Morrison assumed the top job.

Sharma is Turnbull’s preferred candidate for the seat.

Wentworth byelection: John Hewson urges Liberal 'drubbing' over climate change Read more

The Liberals have struggled during the campaign, with Sharma under pressure over climate change and the treatment of refugees in offshore detention – two resonant issues in the socially progressive electorate.

Locals are also angry about Turnbull’s treatment by parliamentary colleagues. Seat polls – while not always reliable – suggest the Liberals will struggle to hold the seat.

On Monday night, Alex Turnbull, again stepped into the campaign to try to convince electorate of Wentworth to vote against the Liberals and instead give their vote to Labor’s Tim Murray, who he described as “an old friend” and “an unusual pick for the Labor party”.

Alex Turnbull said the Liberals’ inability to formulate a coherent policy on energy and climate change was the main driver for speaking out and denied he was damaging his father’s legacy.

“Yes, there are lot’s of other issues but with this it’s just science and economics. If you can’t get to somewhere sensible on something quite boring and dry[ as climate change policy] , you have a problematic party,” he said.

He said it was essential to have policy now. “If you want to decarbonise, it might take 10 to 15 years but you have to do it in a sensible manner.”

Alex Turnbull said his decision to speak out against the Liberals was driven by his friends in the Republican party in the US.

“Their party was taken over and now it’s very difficult to get it back. In Australia, we are slowly drifting toward culture wars as a platform,” he said, warning that “pandering to extremes was not a winning ticket”.

Over the weekend Morrison attempted to warn Wentworth voters against generating a boilover that would cost the government its one-seat majority in the lower house.

“Whatever concerns you may have about events of several weeks ago, if you don’t vote for the Liberal candidate, then you risk a hung parliament,” the prime minister said at the weekend. “You risk creating unnecessary uncertainty in our economy and the stability of our government more broadly.”

With parliament resuming in Canberra on Monday, Morrison drew on a Forrest Gump analogy to urge Wentworth voters against lodging a protest vote, telling reporters: “With the Liberal party you know what you are going to get.

“With the Labor party you never know what you are going to get and with independents you certainly don’t know what you are ever going to get.

“It is like the good old box of chocolates – you never know what you are going to get when it comes to voting independent – we have seen that in this parliament too often.”

Abbott on Monday also moved to characterise the high-profile independent candidate Kerryn Phelps as “effectively a vote for Bill Shorten”.

Wentworth byelection: could Tim Murray deliver Labor's sweetest victory? Read more

“The official Labor candidate is some bloke but the real Labor candidate is Kerryn Phelps and that’s why if you vote for Kerryn Phelps you are really voting for Bill Shorten and it is important that people understand that,” Abbott said.

He shrugged off the latest batch of opinion polls suggesting the Coalition is still facing a negative backlash after the leadership change.

Abbott said Morrison was doing a good job, speaking in “short clear sentences” and the new prime minister was “a tribal Liberal, and that is what we want – we want someone who believes in our cause and regards themselves as a creature of our party”.