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Outgoing MP Vince Catania admits the West Australian Nationals face an uphill battle to retain his seat as Labor eyes the chance to extend its parliamentary dominance.
Mr Catania, 45, has announced he will quit politics after having been a member of state parliament since 2005. He will formally step down as the member for North West Central in August.
It is another blow for the opposition Liberal-Nationals alliance, who were reduced to six lower house MPs after Labor's crushing victory at last year's state election.
Mr Catania on Friday said he had "nothing left in the tank" and wanted more time with his wife and five teenage children.
But he acknowledged the Nationals would struggle to hold the vast rural electorate which he had retained by just 259 votes after suffering an 8.4 per cent swing to Labor.
"It's going to be a very difficult by-election," he told reporters.
"But we'll get the right candidate, a candidate that's going to fight for a lot of the issues. This is a real test for the McGowan government."
Mr Catania initially served as a Labor MP in the upper house before switching to the Legislative Assembly.
His defection to the Nationals in 2009 earned the ire of Labor MPs, with Education Minister Sue Ellery using a news conference on Friday to deliver a parting shot.
"He has form of leaving parties when it is the worst time to leave those parties," she said.
"From my personal point of view, loyalty and fidelity matters."
The by-election will hand Premier Mark McGowan's Labor government the opportunity to extend its reach into the regions, having last year snatched two Nationals-held seats.
Opposition Leader Mia Davies said Mr Catania had informed her of his intentions several weeks earlier but insisted a replacement candidate was yet to be picked.
Ms Davies threatened to resign as Nationals leader in 2019 in response to Mr Catania plotting to challenge the position of her then-deputy Jacqui Boydell.
She said the party would campaign on issues with health, education and youth crime.
"Obviously it'll be a challenge," she said.
"It's a very unique circumstance in Western Australia at the moment but ... the people of North West Central understand they've been neglected by this government for five years."