David Littleproud stepped down from the regional party's top job on Tuesday, declaring he was "buggered" and didn't have the energy to continue as leader.
Party members will meet on Wednesday morning to choose a new leader, with four contenders expected to run.
Deputy leader Kevin Hogan, former deputy prime minister Michael McCormack and senators Bridget McKenzie and Matt Canavan are all thought to be canvassing for votes.
Senator McKenzie took to social media on Wednesday to announce her tilt, pledging to work every day to secure Australia's future.
"Today I am seeking to lead the National Party," she said.
"We face enormous challenges as a country and the Nationals have a critical role in protecting and growing our nation."
Senator Canavan, who was the first to declare his intention to run, said he thought he'd made a good case to his colleagues.
"I've worked hard for the party," he told reporters on his way into Parliament House.
"I think I have delivered results in the past, particularly when I was minister, I helped deliver the net zero policy."
Fellow Queensland senator Susan McDonald said she'd put her hand up as deputy leader and praised Senator Canavan, saying he was an "incredible team player".
Mr Littleproud's announcement he was stepping down took many in the party by surprise.
The Queensland MP will continue representing his seat of Maranoa and has said he's willing to remain on the coalition front bench.
Mr Littleproud said he was feeling like a "million dollars" after stepping down, adding he'd had his best sleep in eight months.
"For the first time today, I've had a smile," he told reporters on Wednesday morning.
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 failed to win back 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.
Firebrand Northern Territory senator Jacinta Nampijinpa Price also 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".
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