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A vote could be held at the next caucus meeting on Tuesday, but if Mr Hide accepts he has lost majority support to the former National Party leader, he could resign earlier.
The Act board is due to meet on Saturday in Auckland, and if Mr Hide has not resigned by then, that could be the logical place for him to accept defeat.
It is most likely he will remain in Parliament and not force a by-election. It is not clear whether he will remain as an Act MP or an independent MP for Epsom.
Dr Brash would become leader outside Parliament. Prime Minister John Key said from London yesterday that if Mr Hide was rolled as leader he would be able to keep his ministerial posts - Local Government, Regulatory Reform and Associate Education.
Dr Brash hosted Ms Calvert and Act founder Sir Roger Douglas together yesterday. It is understood Ms Calvert, whose vote is crucial in a caucus vote on the leadership, has switched from backing Mr Hide to supporting Dr Brash.
One of the issues they discussed was Dr Brash's position on the Treaty of Waitangi and Maori issues. Dr Brash said last night he and Ms Calvert, who was a vociferous opponent of the foreshore and seabed reform Bill, saw eye-to-eye on those issues.
The extraordinary events of the past week have played out in public, after Dr Brash made public his wish to take the leadership of the Act party, and threatened to set up his own right-wing party if his bid failed.
Dunedin-based Ms Calvert entered Parliament in September last year after former MP David Garrett quit in disgrace, following revelations he had committed identity theft and had a previous assault conviction.
Ms Calvert said on Sunday she backed Mr Hide and would vote for him over Dr Brash, and on Tuesday she repeated that position. But yesterday after the meeting, when asked if her position was still the same, she said: "I'm not prepared to make any comment."
Sir Roger and MP Heather Roy have been in the Brash camp, having had major differences with Mr Hide's style of leadership. A vote from Ms Calvert would give Dr Brash the required numbers by a 3-2 majority.
Last night Mr Hide said he did not know if he had majority support in caucus any more.
"The caucus and the board need to make their own decision ... What I've always done is accepted that I'm the leader while I have the support of the caucus and the board," he told Campbell Live last night.
Even if he knew he had the numbers, he said he would not proclaim victory because he was not "that arrogant, actually, because I serve at the pleasure of the party".
If he was rolled, Mr Hide said he would "support John Key and work to honour the confidence and supply agreement".
"But Don Brash has been very clear about my future ... he said there's no room for me in Parliament."
Dr Brash several times referred to Act as though he already belonged to the party.
"If I can't get 5% [of the vote] I shouldn't be trying to do this. Quite frankly, we should be getting much more than 5%," he said.
Asked why people should vote for Act in Epsom when he was so critical of National, he said: "Frankly, we are the only party in Parliament ... even remotely akin to what National stands for."
Dr Brash reiterated yesterday he would like to see former Auckland mayor John Banks stand for Act in Epsom. Dr Brash said he would better serve the party by gathering support across the country, rather than be beholden to an electorate, although he said he would still likely stand in an electorate seat.
Once it is clear Dr Brash has the numbers, he is expected to apply for membership of the Act party.
Party president Chris Simmons has said the board was unlikely to oppose the caucus choice of leader, even though it had the constitutional power to do so.