Walukagga, a singer and first-time contender, had been selected by the National Unity Platform (NUP) as its official candidate over incumbent MP Medard Sseggona in party primaries.
Sseggona, who publicly expressed dissatisfaction with the process, opted to seek re-election as an independent candidate.
The development dramatically reconfigures the contest and is likely to trigger political fallout within NUP, whose internal processes had already sparked friction in the constituency.
More details to follow.