The majority of MPs seem determined to reject another veto by Mr Hashemi
Iraq will not be able to hold parliamentary elections before the end of January as required by the country's constitution, electoral officials say.
The head of the Independent Electoral Commission of Iraq, Faraj al-Haidari, said the possibility was now "over".
Iraq's Sunni Arab vice-president had vetoed a draft of the electoral bill passed by parliament this month.
Tariq al-Hashemi has now indicated he will also oppose an amended version agreed on Monday.
The latest version would see more seats allocated to Kurds and fewer to Sunnis.
The vote is seen as a prerequisite to the US meeting its goal of pulling out combat troops by August next year, and withdrawing fully by 2011.
Last week, Mr Hashemi vetoed a previous version of the election law, which was passed after months of debate in parliament, because he wanted more representation for the millions of Iraqis living abroad.
2003: US appoints Governing Council
2004: Governing Council elects interim government
Aug 2004: National conference elects Interim National Assembly
Jan 2005: First general elections for Transitional National Assembly and provincial councils
Dec 2005: General elections for first full-term government and new parliament, the Council of Representatives
Jan 2009: Polls for provincial councils - key test of security gains
The main Shia and Kurdish factions voted for a compromise solution on Monday, which passed despite continuing opposition from Sunnis.
But a statement from Mr Hashemi's office condemned the amendment as "unfair and unconstitutional".
The statement said he would "deal with the new law as he did with the previous one to protect the national interest and bolster democracy", though it declined to say whether he would veto it again.
President Jalal Talabani, a Kurd, and the US ambassador to Iraq, Christopher Hill, voiced support for the amended version.
"I would just ask people to study this very carefully and to understand that any law does involve some trade-offs," Mr Hill said.
Parliament can override a second veto with a three-fifths majority, and correspondents say most MPs seem determined to do so.
In theory, the law must be approved 60 days before any election can be held, making Tuesday the deadline if the proposed date of 23 January is to be met.
However, the Independent Electoral Commission said that whatever happened, it was now too late to organise a vote that month.
"In all cases the possibility of holding the vote in January is over," Mr Haidari told Reuters news agency.
The commission will wait for the dispute to end before setting a date.