Bemba, one of six hopefuls excluded by the election commission from the December 23 vote, told magazine Jeune Afrique the country is still “very far” from holding a democratic election.
“But if the elections take place under (democratic) conditions, and if the opposition unites behind one candidate, then yes... I will support that person and I will make them win,” he said.
Bemba has previously accused President Joseph Kabila of trying to hand-pick a successor by eliminating serious rivals, branding the vote a “parody” after he was barred from contesting earlier this month.
The former Belgian colony has not seen a peaceful transition of power since 1960.
Kabila — in power since 2001 despite his second and final term ending two years ago — said in August he will not run again after keeping silent on the issue for months, fuelling tension and deadly unrest in the volatile central African nation.
The president has named his former interior minister Ramazani Shadary as his chosen successor.
In an interview headlined “Kabila was scared of me”, Bemba said any call to boycott the election “would not be my personal decision, but that of the opposition”.
Bemba, who was vice-president of an interim government between 2003 and 2006, had appealed the decision to exclude him from running in the election.
But the Constitutional Court upheld the ban, citing his conviction by the International Criminal Court (ICC) for bribing witnesses at his war crimes trial.
Bemba told the magazine that the ICC prohibits him from commenting on the case, but added that bribing witnesses did not constitute corruption.
“In DR Congo’s penal code, corruption and perjury are defined by two different articles of law,” he said. — AFP