Act leader David Seymour has been urged to "take a breath" and vote against the SkyCity convention centre, despite Government pressure.
Mr Seymour could hold the deciding vote on the $400 million deal for SkyCity to build a national convention centre in Auckland.
Parliament will get a chance to overturn the law giving SkyCity concessions in exchange for building a national convention centre.
It would pass if two things happened: Winston Peters wins the Northland byelection and Act supports the repeal.
Last night Mr Seymour indicated to the Herald that he was not opposed to the repeal, which is in a private members bill in the name of NZ First deputy Tracey Martin.
But, after a subsequent phone call from Economic Development Minister Steven Joyce, he said it would be a big move to overrule the "sanctity of contract" between the Government and the casino.
This morning Ms Martin urged the Act leader to "take a breath and vote what you think".
"The world won't end. They can't get rid of you, because you are an elected representative," Ms Martin said.
"It would be farcical if it wasn't so awful ... we have an elected representative being browbeaten by members of the Government, by ministers - because what would you call that phone call from Steven Joyce making sure that David Seymour knew his place?"
Ms Martin's bill simply repeals the New Zealand International Convention Centre Act 2013, and was introduced to Parliament after being drawn out in the private members' ballot yesterday.
Mr Seymour has expressed concern at the convention centre deal, which was initially negotiated last term before he entered Parliament last year.
After being asked for comment last night, Mr Joyce called Mr Seymour and told the Herald he had been assured by Mr Seymour that he would not be supporting the repeal bill.
"He has basically landed on the position that a contract is a contract and he is certainly not going to do anything to put that at risk."
But when Mr Seymour was asked again by the Herald if he would rule out supporting the bill, he said: "No, I'm not ruling it out. But the indications I've had so far about what it would actually mean to go through with the kind of bill Tracey Martin has proposed, about the sanctity of contract and certainty about the future are important too and that is something I have got to weigh up."
He said he could understand why Mr Joyce couldn't imagine Mr Seymour doing such a thing, given the consequences, "and I suspect he's probably right".
In the event that Mr Seymour did support the repeal, and if Mr Peters wins the Northland byelection, which would allow New Zealand First to gain another list MP, the bill would have enough support to pass, by 61 votes to 60.
If National loses Northland, it would have 59 MPs and, assuming United Future's Peter Dunne continued to support the convention centre, it could be assured of only 60 votes to support the existing legislation, in a 121-seat Parliament.Opposition to the existing law by Labour, the Greens, New Zealand First, the Maori Party and Act would be 61.
The original bill passed into law in November with a narrow margin of 61 votes to 59.
The byelection follows the resignation of National's Mike Sabin as Northland MP and will be held at the end of next week, March 28.
Labour leader Andrew Little has given his implicit blessing to Labour supporters to back Mr Peters.
Mr Peters is a list MP and if he were to win an electorate seat, he can resign as a list MP to become an electorate MP and New Zealand First would get another list MP, taking its caucus from the present 11 to 12.
A spokesman for SkyCity said the company declined to comment.