NAIROBI, June 13 -- Rigathi Gachagua, the deputy president William Ruto sacked in a dramatic impeachment, has retreated into seclusion to attempt the one thing that could cost Kenya's leader a second term: welding a fractured opposition into a single challenger for 2027.

Gachagua announced a 45-day withdrawal from the rally circuit to his rural home at Wamunyoro, in the Mount Kenya heartland, where he said he would spend the time in consultations aimed at uniting Ruto's rivals behind one presidential candidate rather than the crowded field that has split the opposition vote before.

The arithmetic he is chasing is daunting and specific. Gachagua has spoken of mobilising millions of Kenyans, courting the allies of former president Uhuru K...