The All Progressives Congress (APC) swept to victory in the governorship and House of Assembly elections conducted across the country.
The party of Nigeria's incoming president has won a landslide in elections for powerful state governors, ending the former ruling party's dominance.
The All Progressives Congress (APC) won 19 of the 28 governor posts in results declared from Saturday's elections.
It is the biggest defeat for the Peoples Democratic Party (PDP) since military ruled ended in 1999.
It lost the presidential poll last month for the first time.
APC leader Muhammadu Buhari will be inaugurated on 29 May, after he defeated President Goodluck Jonathan.
The PDP's decline has been reinforced by results from Saturday's elections for governors and state assemblies, reports the BBC's Bashir Saad Abdullahi from the capital, Abuja.
The party lost for the first time in key northern states like Katsina and Kaduna, the home of outgoing Vice-President Namadi Sambo.
It also lost control of Adamawa, one of three states badly hit by militant Islamist group Boko Haram's insurgency.
However, the party managed to hold on to Rivers State, Nigeria's oil hub.
The PDP won eight governorships, mostly in the south-east.
Taraba is the only result still outstanding, while the election in Imo state was declared inconclusive, because the number of spoilt ballot papers was larger than the margin of victory.
Elections were not held in seven states.
Nigeria's 36 governors enjoy wide powers and are extremely influential. Some, especially in oil-producing areas, control bigger budgets than those of national governments in some neighbouring West African countries.
The Independent National Electoral Commission (Inec) reported 66 violent incidents at polling stations - higher than in the presidential poll.
Most of the violence took place in Rivers and other southern states, where voting at some polling stations had to be extended.
On Sunday, Inec also announced that the APC had retained power in Lagos state.
It means that when Gen. Buhari takes over the presidency it will be the first time that the party of the president has controlled Lagos - the commercial capital - since the advent of multi-party democracy in 1999.
Our correspondent says the PDP will have to go through a period of introspection, and then rebuild itself under a new leader.
The party was in power for 16 years, and voters felt that it was time for change, he says.
It had been hit by the defection of governors and MPs to the APC ahead of the elections.
The PDP also faced widespread allegations of corruption during its rule, and was accused of failing to enough to end an Islamist-led insurgency in the north-east where it suffered huge defeats.