Former prime minister John Howard says high-profile Liberals have great confidence in Mayo candidate Georgina Downer.
But Mr Howard, who campaigned with Ms Downer in South Australia on Wednesday, admits winning back the prized seat for the party will be tough.
"She realises she's got to fight for every vote," he told reporters.
Mr Howard said visits from Prime Minister Malcolm Turnbull and senior government ministers during the lengthy by-election campaign demonstrate the party's faith in their blue-blood candidate.
"It's an expression of confidence - if they thought she was a hopeless candidate, they wouldn't come," he said.
"Good candidates attract high-profile supporters."
Mr Howard travelled to Goolwa, south of Adelaide, to meet voters and also had a scheduled stop at nearby Victor Harbor.
He warned that crossbencher Rebekha Sharkie, who won the seat in 2016 as a member of the Nick Xenophon Team and is now part of the rebadged Centre Alliance, had a history of voting with the opposition.
"The former member, who held it for the last couple years, has, on occasion, voted with the Labor Party on issues that are quite important to our border security and taxation support for small to medium-sized businesses," he said.
In an open letter to Mayo voters, Mr Howard said a vote for Ms Sharkie would be helpful to Bill Shorten and Labor.
"If elected, we don't know what Ms Sharkie will vote with Labor on next," it read.
Ms Downer said voters throughout the campaign had raised concerns about the voting record of Ms Sharkie.
"They're asking the question, with that type of record and with the views she's been holding that do align with so many of the Labor Party's policies, what does she owe Bill Shorten?"
"And if she were to be re-elected, what way would she vote in subsequent months in the parliament?"
Ms Sharkie was forced to resign from federal parliament earlier this year after being caught up in the dual-citizenship saga.
She has repeatedly rejected coalition criticisms that she more often backs the opposition in parliament.
If Ms Downer wins she will follow in the footsteps of her father, Alexander who held Mayo from 1984 to 2008 and served as foreign minister in the Howard government.
Voters will go to the polls on July 28.
