The Maori Party could have the casting vote on an attempt by Labour and New Zealand First to revive the so-called "waka-jumping act".
NZ First secured Government support for re-introducing the Electoral Integrity Act as part of their confidence and supply agreement announced last week.
The contentious party-hopping law was passed by the Labour-Alliance coalition in 2001 but included a sunset clause, which meant it expired at this election.
NZ First leader Winston Peters said his party had opposed the sunset clause and "to be consistent", supported the re-introduction of the bill.
The law required MPs who wanted to leave their party to resign from Parliament, and allowed parties to expel MPs who flouted policy. It followed the high-profile case of Alamein Kopu, who left the Alliance in 1997 but stayed in Parliament and voted with National, and the public fracturing of NZ First, which lost nine MPs in 1998.
But the re-introduced bill could fail at its first hurdle, with National, Act, United Future and the Greens combining to oppose it.
That would leave Labour, Jim Anderton and NZ First with 58 votes, needing the Maori Party's four votes before the legislation could continue.
Maori Party co-leader Tariana Turia said yesterday the party did not yet have a position on the issue, but would discuss it at caucus.
Opponents say the anti-party-hopping law forces MPs to toe the party line regardless of what their conscience or electorate says.
Act leader Rodney Hide said the bill was "dopey law".
"If MPs choose to exercise their conscience and leave a party, then they should be entitled to," he said.
"Winston Peters has now got to use the law to keep his MPs in the tent, that's how disgruntled they are with him taking the baubles of office."
National Party deputy leader Gerry Brownlee dismissed the bill as "Winston Peters' handcuff bill".
"He has to put a handcuff on his own team to make sure they don't bail out."
Mr Peters rejected suggestions that he was simply seeking to discipline his own caucus. "There's a phrase for that - they're on their back talking fustian."
Green Party co-leader Rod Donald said the anti-party-hopping law was "anti-democratic and draconian".
"You've got to ask why people in Labour and NZ First think some of their MPs are going to desert their parties," he said.
United Future leader Peter Dunne could not be reached for comment yesterday but opposed the original introduction of the law, calling it "a mandatory shackle on people's consciences".
- HERALD ON SUNDAY