Sussan Ley’s embattled leadership has suffered a major blow, with key backer and Queensland senator James McGrath resigning from the frontbench in order to back Angus Taylor.

Taylor’s supporters are growing increasingly confident the former shadow defence minister will win Friday’s Liberal leadership ballot, set down for 9am at Parliament House.

McGrath’s move to quit was quickly followed by opposition Senate leader and foreign affairs spokesperson, Michaelia Cash, as she moved her support to Taylor.

After resigning from shadow cabinet on Wednesday night, Taylor confirmed plans to challenge Ley in a social media video posted on Thursday morning, amid a wave of resignations from the shadow frontbench.

“The Labor government has failed and the Liberal party has lost its way,” he said.

“I’m running to be the leader of the Liberal party because I believe that Australia is worth fighting for. I believe we need strong and decisive leadership that gives Australians clarity, courage and confidence in providing a vision for the future.

“I’m committing myself to the cause of restoring our party, so that it can be the party that Australians expect and deserve, because we’re running out of time and Australia is worth fighting for.”

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The video was posted after two of Taylor’s allies – Jess Collins and Phil Thompson – handed a letter to Ley on Thursday morning requesting a motion to spill the leadership.

Senior conservatives Jonno Duniam and James Paterson resigned at about 10am, joining fellow frontbenchers Matt O’Sullivan and Claire Chandler, in a coordinated move designed to build pressure on Ley.

McGrath, a former party strategist and powerbroker, quit about 3pm. His vote could be decisive in the ballot, expected to be settled with a narrow margin.

“We must take the fight to Labor and I have made this decision with that at the forefront,” McGrath said in a statement.

“Whether on net zero, immigration, violent Islamic extremism, cost of living or keeping Australians safe, Labor and Anthony Albanese have failed.”

Thompson cited poor polling for his decision.

“We’ve seen that the people are quite upset. Well, the way that we act is by change, and nine months is enough time to be able to turn that around, and that hasn’t happened.”

Under party conventions, two or more Liberal MPs can request a special party room meeting to vote on a leadership spill.

If a majority of MPs support the spill motion, a secret ballot then determines the leader.

Ley has not spoken publicly about the challenge, but attended question time and spoke on the government’s Closing the Gap agenda in parliament. She has posted a series of positive messages to social media, promising “a better future” for the country.

After days of speculation, Taylor late on Wednesday night resigned from shadow cabinet to set up a challenge against the party’s first female leader.

“I don’t believe Sussan Ley is in a position to be able to lead the party as it needs to be led,” he said in an evening press conference.

“What we need right now is strong leadership, clear direction and a courageous focus on our values, and the first two priorities … should be protecting our way of life and restoring our standard of living.”

The moderate-aligned Ley defeated the conservative Taylor 29 votes to 25 to win the leadership after the 2025 election.

While the two camps are confident their candidate has majority support of the 51-member party room, both admit the numbers are tight.

Allies of Ley and Taylor were out on Thursday morning voicing support for their preferred leader.

The shadow attorney general, Andrew Wallace, was confident Ley would survive if a challenge materialised.

“I think this, this leadership challenge, if it ends up being that, has been undercooked,” he said.

“I think it’s been unprepared or ill-prepared. I think Sussan has the numbers. I think that the majority of the party room believes that Sussan hasn’t been given a fair go.”

Jockeying is also under way for the Liberal deputy leadership.

Incumbent Ted O’Brien faces an uphill challenge if Ley is rolled, with Victorian Jane Hume emerging as a possible frontrunner for the job under Taylor. O’Brien was speaking to colleagues on Thursday morning.

Other possible contenders include Flinders MP Zoe McKenzie, and frontbenchers Dan Tehan and Melissa McIntosh, though neither have resigned from their roles to contest the race, so far.

Goldstein MP Tim Wilson is also being discussed.

But there is a view among many Liberals that Taylor should have a female deputy.

One source said gender diversity would help the Liberals avoid the perception “that two blokes had rolled Sussan Ley and taken over the party again”.

Another source was blunt about O’Brien’s position, telling Guardian Australia “there is no appetite to keep Ted”.