Martin Fayulu, a little-known MP unexpectedly selected as unity candidate in talks in Geneva, insisted the accord was not dead, and urged dissenting leaders to return to it.
With the ballot looming fast, six other opposition leaders agreed on Sunday to rally behind Fayulu as their joint champion.
The December 23 elections are a watershed for the volatile, poverty-stricken country, which President Joseph Kabila has ruled with an iron fist since 2001.
A DR Congo legislator chosen by opposition leaders to be their champion in next month's presidential election pleaded for unity on Tuesday after their historic deal was shot down by party activists.
Martin Fayulu, a little-known MP unexpectedly selected as unity candidate in talks in Geneva, insisted the accord was not dead, and urged dissenting leaders to return to it.
"The agreement is still alive," Fayulu said on the television channel TV5Monde after two other party leaders backed away from the accord just a day after signing it.
"I urge my brothers to overcome partisan considerations and to give priority to the nation's higher interests," he said later in a tweet. "It's never too late to do the right thing."
December vote
The December 23 elections, also unfolding at legislative and municipal level, are a watershed for the volatile, poverty-stricken country, which President Joseph Kabila has ruled with an iron fist since 2001.
Foreign observers fear tensions around the vote could trigger a violent flare up.
With the ballot looming fast, six other opposition leaders agreed on Sunday to rally behind Fayulu as their joint champion.
The move sought to overcome internal differences to boost the opposition's chances of defeating Kabila's hand-picked candidate, Emmanuel Ramazani Shadary, a loyalist and hardline former interior minister.
But less than a day after the Geneva deal was signed, the accord came under fire from activists in two parties and within hours their leaders announced they were scrapping their support.
One is the Union for Democracy and Social Progress (UDPS), DRC's oldest and biggest opposition party, led by Felix Tshisekedi, who was widely seen as the frontrunner. The other is the smaller Union for the Congolese Nation (UNC), led by Vital Kamerhe.
Fayulu, in his TV interview, said the U-turn was "extremely serious".
"What message are we sending to children and the public, especially we in the opposition who are always saying that the Kabila government does not respect the constitution?" he asked.
But a UDPS official in Brussels, where Tshisekedi went after Geneva, appeared resolute.
"For the time being, the dialogue is off," he told AFP.
Fayulu support
As the standoff continued, support for Fayulu came from an unexpected quarter with leading DRC singer-songwriter Fally Ipupa, who has won a slew of international awards, offering his moral support.
"I am very very proud of him. And it's a very good thing that the politicians get their act together for the good of the country," he told TV5Monde. "Anyway, I wish him good luck."
Only four opposition leaders had their candidacies approved by the election board, and the Geneva agreement requires that the other three withdraw out of "solidarity" with the unity candidate —a move angrily rejected by the UDPS and UNC activists.
An opinion poll published in late October showed Tshisekedi and Kamerhe were leading the pack ahead of the December election followed by Ramazani Shadary.
According to the well-informed French-language magazine Jeune Afrique, the Geneva accord also outlined a strategy in the event that the presidential ballot did not take place, suggesting opposition leaders would hold a conference to discuss "new options".
This element of the text incensed UDPS supporters, who suspected a plot to manipulate strategy by two influential opposition figures who were banned from running in December's poll: former warlord Jean-Pierre Bemba and ex-provincial governor Moise Katumbi.
Both attended the Geneva talks.
"We thought that our 'barred' friends were not being sincere with us, that they wanted to draw us into a boycott, which is a dangerous path for DRC," UDPS secretary general Jean-Marc Kabund told AFP.
"We had to spill our blood before Kabila agreed to hold these elections. And we're going to sacrifice this effort? We can't accept any of that," he said.
Electronic voting
In another rift with fellow opposition parties, the UDPS has also said it will accept the use of electronic voting machines if this means the ballot can go ahead. Critics, including Fayulu, oppose the machines, seeing them as an invitation to fraud.
The squabbles have predictably been fodder for Kabila's side, who on Twitter said the divisions represented "the nth betrayal of our people by the leaders of the Congolese opposition."
The pro-democracy anti-Kabila movement, Lucha, voiced its dismay.
"Our only choice is to forge change by ourselves," it said. "We've had enough! Our Congo deserves better than this."