* March 4 election the first since 2007-08 violence
* Kenyan politics driven by numbers, not ideology
* Rivals Kenyatta, Ruto face trial at ICC
By Duncan Miriri
NAIROBI, Dec 4 (Reuters) - Kenyan Prime Minister Raila Odinga said on Tuesday he would join forces with the country's vice president in next year's election, to challenge another alliance headed by two senior politicians facing charges of crimes against humanity.
Odinga, Vice President Kalonzo Musyoka and several other cabinet ministers danced in front of thousands of supporters in the Kenyan capital to launch their electoral pact.
The March 4 poll will be the first under a new constitution and the first since the 2007 vote that triggered ethnic killings across the country.
The main challenge to the Odinga-Musyoka ticket will be the alliance formed on Sunday between deputy Prime Minister Uhuru Kenyatta and former cabinet minister William Ruto.
Both Kenyatta and Ruto have been indicted by the International Criminal Court for allegedly inciting the post-election violence that convulsed the region's biggest economy.
The formation of the two alliances follows months of horse trading between leading politicians in a country where allegiances tend to follow tribal lines.
Political alliances also tend to be based on the numbers of supporters each side can bring to the union, rather than shared ideology.
"We have created the only kind of coalition that can work, a coalition of the willing," Odinga said in a statement launching the Coalition for Reform and Democracy (CORD).
The deal unites Odinga's Orange Democratic Movement (ODM) party, Musyoka's Wiper party and 12 other political movements.
Commentators said Musyoka had little choice but to enter his alliance after talks broke down with Kenyatta-Ruto ticket, which brought together the powerful Kikuyu and the Kalenjin communities.
Both Musyoka, who comes from the Kamba ethnic community, and Odinga, a Luo, ran for president in the last election. Their rivalry carried on in the coalition government brokered by former U.N. chief Kofi Annan to end the post-election fighting.
They bickered over who had the most senior position in the government and were also at odds over the ICC's involvement in investigating the violence. Odinga supported it while Musyoka shuttled around Africa lobbying governments to back a referral of the cases back home.
"Whatever they came up with can only be summed up as political nonsense for now," said Mutahi Ngunyi, a Nairobi-based political analyst.
The CORD parties said their delegates would choose which man would stand as their presidential candidate at a later date. (Editing by Richard Lough and Andrew Heavens)