Venezuelans are due to vote on a proposal that would allow President Hugo Chavez and other elected officials to seek as many terms as they wish, BBC reported.
A "yes" vote in Sunday's referendum would allow Mr Chavez to stay in office after his current term ends in 2012.
Mr Chavez has said the constitutional amendment is needed for the future of Venezuela's socialist revolution.
But critics say it is designed to concentrate power in the president's hands for decades to come.
A proposal to end presidential term limits was one of a package of 69 constitutional changes narrowly rejected in a 2007 referendum.
This year, however, the right of re-election has been extended to local governors and state politicians.
Some analysts say this change could make the difference for Mr Chavez, as many local governors are said to back the measure this time around, the BBC's Will Grant reports from Caracas.
But the opposition is adamant that the proposal has been rejected once and should not be back under discussion, he says.
Mr Chavez celebrated 10 years in power earlier this month. His current term is due to end in 2012.
"Ten years is nothing," Mr Chavez said at a news conference on Saturday. "I don't know what they're complaining about."
"On Monday I'll wake up looking beyond 2013, and that will give me more confidence in what we're doing."
Mr Chavez also said that the expulsion on Friday of a Spanish deputy of the European parliament, Luis Herrero, would not affect Venezuela's relationship with Spain.
Mr Herrero had been invited by a Venezuelan opposition party to observe Sunday's referendum and had criticised a decision to keep polls open for two hours longer than usual.