Three of the country's four presidents -- Jomo Kenyatta, his son Uhuru, and Mwai Kibaki -- are Kikuyu.
The economically powerful community has always backed its own people -- a widespread phenomenon in Kenya, where the tribal vote has played a big role in previous elections.
Politicians exploited ethnic divides to deadly effect in 2007-2008, when Kenya witnessed a vicious bout of post-poll violence that pitted mainly Kikuyus against Luos and Kalenjins, leaving more than 1,100 dead.
But this year, Kikuyus -- who account for around six million of Kenya's 22 million voters -- will have to choose between Ruto, a Kalenjin and Odinga, a Luo.
The alliance between Kenyatta and Odinga has ruffled feathers in Mount Kenya, where some have turned on their former leader, accusing him of abandoning "kihoto" (the Kikuyu concept of justice) by sidelining his deputy.
"We as the Kikuyus cannot be told who to support, Uhuru is a traitor," George Mwaura, a 38-year-old motorcycle taxi driver, said at the Kirinyaga rally.
But Odinga's team insists that the 77-year-old will win a majority in the region, pointing to the selection of Kikuyu lawyer Martha Karua as his running mate.
The choice of Karua, a former justice minister nicknamed the "iron lady", has "energised and excited the mountain", said David Murathe, a prominent Kikuyu and vice-chairman of the Jubilee party.
Ruto's running mate Rigathi Gachagua is also a Kikuyu who once served as Kenyatta's personal aide, but Murathe said he lacked Karua's credentials as a key player in Kenya's pro-democracy movement.
Even accounting for a worst-case scenario, the Odinga-Karua ticket could expect to secure at least 60 percent of Mount Kenya's votes, Murathe told AFP.
"I would not be surprised if Ruto gets the shock of his life" when the results are announced, he said.
Academic Macharia Munene also testified to the presence of the "Martha effect", telling AFP: "There was a time it appeared to be all Ruto, about six-seven months ago.
"It's declining, that overwhelming support. It's been eroded," he said.
"Whether the erosion is big enough to turn the tables is another matter."