In 2007, Warren Mundine wrote in a national newspaper that he "hated John Howard" and wanted to "see him voted out at the next federal election."

Key points: John Howard campaigned in Gilmore on behalf of Warren Mundine, who once said he "hated" the former PM

John Howard campaigned in Gilmore on behalf of Warren Mundine, who once said he "hated" the former PM Former Liberal MPs have endorsed the National Party's candidate, Katrina Hodgkinson

Former Liberal MPs have endorsed the National Party's candidate, Katrina Hodgkinson The seat is the most marginal in NSW for the Coalition, which holds it by just 0.7 per cent

But today the former prime minister was in Gilmore — the Coalition's most marginal federal seat in NSW — to formally endorse the Liberal Party candidate.

Mr Mundine said that over the past decade Mr Howard — who at the time described Mr Mundine's language as "juvenile" — had helped change his mind about the Liberal Party.

And Mr Howard — returning the favour — turned to the bible to help give his former foe's comments some context.

"When I was a child I thought as a child," Mr Howard said.

"And when I became an adult I put away my childish views and became and adult.

"Warren's views and political allegiance and personal views have matured as well."

Messy race for Gilmore

As Mr Howard joined the campaign trail today, the contest for Gilmore became a little more complicated.

Despite being handpicked by Prime Minister Scott Morrison, Mr Mundine has not been able to win the support of retiring Liberal member Ann Sudmalis.

It came as another senior Liberal — and former member for Gilmore — Joanna Gash, today officially declared her support for National Party candidate Katrina Hodgkinson.

Katrina Hodgkinson has been officially endorsed by retiring and former Gilmore Liberal MPs Ann Sudmalis and Jo Gash — and former Nationals MP John Sharp. ( Facebook: Katrina Hodgkinson )

Ms Gash said the National Party was a better fit for the seat.

"I am providing choice for people, I believe in choice," she said.

"I don't know Mr Mundine — I know he wasn't a member of the Liberal Party until the day that he was told he was the Liberal candidate here."

Ms Gash, who is also volunteering as Ms Hodgkinson's campaign coordinator, denied she was supporting the Nationals to settle the score.

Mr Mundine's selection meant the local Liberal branch was unable to endorse its own candidate.

"I'm not into those games at all — that is one of the reasons I guess I am doing what I am doing behind the scenes," she said.

"I have no issue with anybody.

"I want to move on."

Ms Hodgkinson said she was glad to have the support.

"Individuals have the right to support whomever they choose and have the right to support me," she said.

Mr Mundine said Ms Gash and Ms Sudmalis's support for Ms Hodgkinson was an open secret.

Coalition's most marginal seat

The Coalition holds Gilmore by just 0.7 per cent, making it its most marginal seat in NSW.

Labor candidate Fiona Phillips has been campaigning for the past two years.

The field is further split with disendorsed Liberal candidate Grant Schultz now running as an independent.

"I really feel for Warren Mundine," Mr Schultz said.

"His treatment by those MPs is appalling,

"If they feel like the Liberal Party does not represent them or their values, then they should do what I did and resign from the party and actually join the Nationals."

The ballot draw has Mr Mundine and Mr Hodgkinson in the bottom two spots.

One Nation failed to field a candidate, despite the party's state leader Mark Latham tweeting his interest in the seat in only weeks ago.