Former Liberal MP and aspiring senator Sarah Henderson is being targeted by religious conservatives in the Victorian branch of the party for being a supporter of former PM Malcolm Turnbull, gay marriage and abortion.

Key points: Sarah Henderson is seeking a political comeback after losing her Lower House seat

Sarah Henderson is seeking a political comeback after losing her Lower House seat She's in a tight battle for a Senate seat against Greg Mirabella

She's in a tight battle for a Senate seat against Greg Mirabella Both campaigns have faced secretive smear attacks criticising the candidates

Ms Henderson is one of two frontrunners to replace former Victorian senator Mitch Fifield, who quit politics to become Australia's ambassador to the United Nations.

She has the backing of Prime Minister Scott Morrison and most of her former federal parliamentary colleagues, including Victorian ministers Josh Frydenberg and Alan Tudge.

Despite this, her supporters concede it will be a close race against Greg Mirabella, a past member of the party's administrative committee. He is the husband of former Liberal MP Sophie Mirabella.

Last week, emails to party members attacked Ms Henderson's record on social issues. It included a link to an article criticising her in The Spectator.

Recipients were criticised for reportedly backing Ms Henderson, who the email described as "a Malcolm Turnbull, gay marriage and abortion supporter".

"I am deeply disappointed to hear that principles and beliefs are so easily traded for the lure of political power,'' the email from an anonymous sender stated.

A complaint has been lodged with party officials.

Greg Mirabella, who is married to former MP Sophie Mirabella, poses the greatest threat to Ms Henderson's Senate bid. ( AAP: Simon Dallinger )

In recent years, there has been a concerted recruitment drive focused on conservative churches and community groups. Former Family First candidates have also joined the party.

There have been fears from some Victorians that the party will become "unelectable'' in the state if it "lurches too hard to the right".

The Liberals were thrashed at November's Victorian state election and hold just 12 of the state's 38 federal seats despite Scott Morrison's election triumph.

Sunday's preselection vote is being viewed internally as a test of factional power within the fractious branch, which has been plagued by infighting.

In the past year, a group of religious conservatives associated with federal vice-president Karina Okotel have split away from a group aligned to Marcus Bastiaan.

One senior source said the numbers were tight and Ms Henderson's opponents, "are working hard to smear the lead candidates with material designed to suppress turnout or influence preference flows".

How the preferences of the other candidates — including defeated Dunkley MP Chris Crewther, Kyle Hoppitt, and former state MP Inga Peulich — flow will be critical.

Scott Morrison has backed Sarah Henderson's bid to return to federal politics. ( ABC News: Marco Catalano )

It is understood supporters of conservative candidates, including Mr Hoppitt, are likely to direct preferences to Mr Mirabella over Ms Henderson.

In recent days, questions have also been raised about Mr Mirabella, regarding where he lives and his directorship of a shell company.

Mr Mirabella's pitch includes representing country Victorians, but his opponents have highlighted he splits his time between Wangaratta and South Melbourne.

The current Liberal Senate team have their electorate offices in Richmond, Moonee Ponds, South Melbourne and East Melbourne.

Ms Henderson is also playing her regional card, having represented Corangamite — that includes Geelong, the Great Ocean Rd and large rural areas — for six years.

Candidates are banned from talking to the media.