Colombia's main opposition Liberal Party says it will reject a bill aimed at giving President Alvaro Uribe the chance of four more years in power.
Uribe says he needs a second term to complete his programme
The party said it opposed changing the law to let Mr Uribe run again in 2006.
Mr Uribe's tough stance against leftist rebels and right-wing paramilitaries has given him approval ratings of 80%, despite human rights groups' concerns.
The Liberals are the biggest party in Congress and their support would have helped the bill to pass easily.
But the party said Mr Uribe's re-election bid would further divide Colombian society.
Camilo Sanchez, who heads the Liberals' leadership team, told the BBC: "There is now no real possibility that this bill can be overwhelmingly approved."
The bill has the support of Colombia's other main party, the Conservatives - but since both parties combined hold fewer than half the seats in Congress, the outcome will depend on the country's many smaller political groupings.
At present, the Colombian constitution allows a president just one four-year term in office.
But the bill due to be debated by the Colombian Congress would allow Mr Uribe and former presidents to stand again for a second term.
Opponents of the move say re-election has generally been shunned by Latin American countries, because of fears of corruption and electoral fraud.
Although countries including Brazil and Venezuela have decided in recent years to overturn laws preventing sitting presidents from running again, the idea remains controversial elsewhere in the region.
A previous attempt to change the law in Mr Uribe's favour was rejected by a Senate commission in October last year.