The Nationals will hold a leadership ballot following the snap resignation of David Littleproud.
A formal vote to elect the new leader will be held on Wednesday morning in Canberra.
Mr Littleproud will remain in parliament representing his Queensland seat of Maranoa, after announcing he was stepping down during an emotional press conference alongside his wife Amelia.
"I'm just buggered," he told reporters at Parliament House on Tuesday afternoon.
"I think someone better can do it, because I don't have the energy."
Outspoken Queensland senator Matt Canavan became the first to confirm he was putting his hand up for the top job.
"I believe I have the best chance to help win the battle for an Australia first-plan that can deliver a better life for all Australians," he wrote in a social media post.
"I will put my case to my Nationals colleagues in the coming days and will respect whatever decision they make.
"I love the Nationals Party and I firmly believe that our pragmatic and grounded approach is what our country now needs."
Deputy leader Kevin Hogan, the party's leader in the Senate Bridget McKenzie, and former leader Michael McCormack are the other potential contenders.
Sources said all four candidates were canvassing support on Tuesday evening.
Consistently poor polling showing Pauline Hanson's One Nation overtaking the coalition in voter support has shaken the Nationals, who fear a wipe-out at the next federal election.
Seats in regional Queensland and NSW are considered particularly at risk, threatening a number of electorates held by the Nationals.
The rural party lost the seat of Calare at the 2025 election to independent Andrew Gee, who quit the Nationals over their opposition to the Indigenous voice to parliament referendum.
Firebrand Northern Territory senator Jacinta Nampijinpa Price defected to the Liberal Party following the federal election defeat.
The move left the Nationals with only four seats in the Senate, below what is typically required to maintain "party status".
Mr Littleproud said he wanted to spend more time with his family but would remain in parliament because he still wanted to represent regional Queensland.
