NASA will hold a rally at Uhuru Park today to announce their final position on Thursday's repeat election.
"The coalition will make the announcement at 2pm Wednesday," Opposition leader Raila Odinga's spokesperson Dennis Onyango said in a statement.
"NASA invites supporters to turn up in large numbers for the crucial announcement," Onyango said.
Raila who will lead the other principals - Musalia Mudavadi and Moses Wetang'ula -
is boycotting the contest against incumbent Uhuru Kenyatta.
He says it will not be free and fair as the election board has made towards reforms he demanded after the August 8 general election was annulled.
Raila had urged his supporters to ensure the fresh election did not take place, repeatedly saying there would be "no election" on October 26.
But on Tuesday
Raila clarified his instructions, saying his supporters should stay away, not demonstrate, in their push for reforms at IEBC.
"We have not told people to demonstrate on the polling day. We have not said that at all. We have told people to stay away," Raila told BBC.
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Raila has called near-daily demonstrations this month to protest against the election body and the draft electoral law, which the President has yet to sign.
The protests have mostly been small and police have used tear gas to quash them.
Last week, police raided homes belonging to wealthy opposition financier Jimi Wanjigi and said they had found a cache of weapons.
Wanjigi said in an emotional interview that the firearms were licensed and that he was being persecuted.