What you need to know:
- He backed Tshisekedi for president in 2018 but has since become a bitter critic, denouncing broken promises and the incumbent's "inability to straighten the country out".
A fourth candidate in this month's Congolese presidential election withdrew in favour of former provincial governor Moise Katumbi on Sunday, bolstering opposition unity behind his bid to defeat incumbent Felix Tshisekedi.
The December 20 vote will be held in one round, seen as benefiting President Tshisekedi's re-election bid.
Some 26 candidates were in the running before the official start of campaigning last month, but former prime minister Augustin Matata Ponyo and two lesser-known candidates have since dropped out and backed Katumbi.
Delly Sesanga, a lawyer and MP from Kasai-Central province and leader of the Envol party, announced he too was backing Katumbi, a wealthy businessman and former governor of the mineral-rich southern Katanga province.
"I have decided, in a surge for national unity... to ally our forces to the candidacy of Moise Katumbi," Sesanga told reporters in the capital Kinshasa and in a message posted on social media.
Sesanga warned the one-round format left the opposition in the central African nation "little choice, to escape the trap of vote dispersal, but to unite".
He backed Tshisekedi for president in 2018 but has since become a bitter critic, denouncing broken promises and the incumbent's "inability to straighten the country out".