The frontrunner in the French presidential election, Nicolas Sarkozy, says he wants both the far right and far left to run in the 22 April poll.
Far-right leader Jean-Marie Le Pen and Olivier Besancenot of the far left are having trouble collecting sufficient signatures to qualify.
Mr Sarkozy said French local officials should not block their candidacies.
Meanwhile, centrist candidate Francois Bayrou has scored 20% in an opinion poll conducted by the LH2 organisation.
Mr Sarkozy is still favourite to beat Socialist Segolene Royal in a runoff.
To become an official candidate, 500 signatures are required from among the 42,000 elected officials in France, most of them mayors of small towns.
But Mr Le Pen, who heads the National Front (FN), and Mr Besancenot, leader of the Communist Revolutionary League, are struggling to collect the signatures, as many mayors seem reluctant to be associated with them.
“My idea of democracy is that candidates can express their ideas,” Mr Sarkozy told French television on Monday. He is still interior minister, but plans to quit that post before the election.
“I fight Mr Le Pen’s ideas. But I will fight to ensure that Mr Besancenot as well as Mr Le Pen can defend their ideas. Democracy mustn’t be hijacked by a small number of people. That isn’t democracy,” he told France 3 TV.
Mr Le Pen, 78, polled 17% in the first round in 2002, and went on to fight the runoff against Jacques Chirac.
Mr Besancenot, 32, a postman by trade, polled 4.25 % in the same election.
Mr Sarkozy’s party, the centre-right UMP, has called on unaffiliated mayors to provide the necessary signatures for candidates who have not yet collected them.
Candidates have until 16 March to submit the required number of signatures.
Some elected officials fear a backlash from their constituents if they are seen to be helping a man—Mr Le Pen—who has been convicted of inciting racial hatred, the BBC’s Emma Jane Kirby reports from Paris.
She says Mr Sarkozy’s gesture may not be quite so altruistic as it seems—if the National Front is not represented in the elections next month, he will be forced to shift his own position to the right. That would alienate some supporters and give more ground to Mr Bayrou.
The LH2 poll shows Mr Le Pen getting 14% in the first round, while Mr Sarkozy leads with 28% and Ms Royal gets 27%. In the second round the figures are 52% for Mr Sarkozy, 48% for Ms Royal.