Preliminary parliamentary poll results revealing big losses for the ruling party show Nigeria "has changed", an analyst has told the BBC.
"It tells a story to every politician: You can no longer take Nigerians for granted," Victor Burubo said.
High-profile PDP casualties include speaker of the lower house Dimeji Bankole and ex-President Olusegun Obasanjo's daughter in the senate.
Despite some violence, observers said Saturday's poll was well-conducted.
The initial vote had to be postponed from 2 April after voting material failed to reach many areas.
Previous elections since the return to civilian rule in 1999 have been marred by widespread fraud and intimidation.
Elections for the presidency and state governorships were also delayed and are now to be held on 16 and 26 April respectively.
Bad luck for president?
With more than 70% of preliminary results announced at a state level, President Goodluck Jonathan's People's Democratic Party (PDP) has suffered significant losses.
The party that has dominated politics since the military returned to barracks has so far taken 59 seats in the 109-member senate and 140 seats in the 360-member House of Representatives.
Correspondents say it is not clear whether the PDP will lose its absolute majority in both houses as voting in some 13-14% of parliamentary constituencies - where polling had begun on 2 April - has been delayed until 26 April.
The party has lost out to two newly formed parties, the opposition Action Congress of Nigeria (ACN) in the south-west and to the Congress for Progressive Change (CPC) in parts of the north.
There was another embarrassing loss for the PDP in the northern state of Katsina where Maryam Yar'Adua, daughter of the late President Umaru Yar'Adua, failed in her bid get into the House of Representatives.
But Mr Burubo, who leads the National Ijaw Council in the southern oil-rich Niger Delta, said the PDP's bad showing on a parliamentary level would not affect the presidential vote.
"I have a feeling that a good number of areas where the PDP has been beaten will still revert to the PDP candidates, Dr Goodluck Jonathan and his running mate Sambo because of who they are are - not just because of the party," he told the BBC's Network Africa programme.
He said Mr Jonathan, who is from the under-developed Niger Delta where inhabitants have felt ignored by politicians, is popular in the region.
His main opponents on the presidential ticket are former anti-corruption campaigner Nuhu Ribadu for the ACN and former military ruler Muhammadu Buhari from the CPC.