The contenders to replace Barnaby Joyce as Deputy PM

Deputy Prime Minister Barnaby Joyce is standing firm in his refusal to resign amid the scandal over the pregnancy of his mistress and former adviser, Vikki Campion.

But some Coalition members have called the position of the Nationals leader "untenable". And there is increasing pressure on Joyce to quit before Prime Minister Malcolm Turnbull leaves the country next week for talks with US President Donald Trump.

Just nine members of the Nationals currently serve in the House of Representatives, and it’s doubtful there is a viable alternative for the role of deputy prime minister among them.

During the Coalition’s time in government the role of deputy PM has always been held by The Nationals leader.

And that’s unlikely to change because of the tenuous nature of the Coalition’s majority in the lower house.

Here are the contenders to be Deputy Prime Minister, should Joyce be pushed aside.

Bridget McKenzie

Bridget McKenzie has been deputy leader of the Nationals for nearly three months, making her the most senior party member apart from Joyce.

The problem for McKenzie is she is a senator.

While there's no specific rule against deputy prime ministers being senators, it is unprecedented.

McKenzie is currently the Minister for Sport, the Minister for Rural Health and the Minister for Regional Communications.

Matt Canavan

Like McKenzie, Matt Canavan is also a senator. But the Minister for Resources faces another roadblock.

The Queenslander is actually a member of the merged Liberal-National Party (LNP), and not the Nationals.

Canavan does sit with the other Nationals in the Senate, at least indicating his preference for the regional party.

Michael McCormack

Serving in parliament since 2010, Michael McCormack is the Minister for Veterans Affairs and Defence Personnel.

Representing the seat of NSW Riverina, McCormack was formerly a newspaper editor in Wagga Wagga, successfully suing his publisher after he was sacked in 2003.

He had a brief stint as minister for small business and a briefer stint as assistant minister for defence.

Luke Hartsuyker

The longest serving National in the House of Representatives, Luke Hartsuyker has represented the NSW North Coast seat of Cowper since 2001.

Hartsuyker is the Assistant Minister for Trade, Tourism and Investment.

Hartsuyker's father is a Dutch immigrant who founded a Netherlands-themed park in Coffs Harbour named The Clog Barn.

David Gillespie

A gastroenterologist by training, David Gillespie replaced Independent Rob Oakeshott in the NSW North Coast seat of Lyne in 2013.

Gillespie is the Assistant Minister for Children and Families and formerly the assistant minister for health.

But Gillespie has some legal headaches to deal with.

Because he owns a shopping complex in Port Macquarie with an Australia Post licensee, the ALP is arguing he has a pecuniary interest in the federal government.

The case is being considered by the High Court.

Darren Chester

Another former journalist, Darren Chester served as minister for infrastructure and transport before getting shuffled out in December last year.

Chester has represented Gippsland in country Victoria since 2008, serving in a variety of minor portfolios in recent years.

He became the first National to declare support for same-sex marriage in June 2015.

He is now a backbencher.