The DAP-K patron warned that the Kenya Kwanza government "had shown signs of mismanaging the nation".
He said the alliance was only good at giving big promises to win votes.
"They promised you Sh50,000 in Hustler Fund but are giving you Sh500. What can you do with that in the current economy?" he said. "This is enough reason you should punish William Ruto and his candidates Mwambu Mabonga (UDA) and Wafula Wakoli (Ford-Kenya)."
And Oparanya exuded confidence that Wamunyinyi would win, saying he was a level-headed politician with vast experience and therefore a better replacement for Moses Wetang'ula, who vacated the seat to seek the National Assembly Speaker position.
"Wamunyinyi is remembered not only in Western but nationally for drafting the Sugar Bill, a legislation that if passed into law will revolutionalise the sugar sector and return it to profitability," said the ODM deputy party leader. "Western's economic mainstay is the sugar sector and a leader who fights to improve the industry is the one we need to push the people's agenda."
The two toured the area three days after Prime Cabinet Secretary Musalia Mudavadi campaigned for Wakoli.
"Wakoli is young, energetic and well acquainted with what is required of a senator by virtue of serving as former senator Wetang'ula's personal assistant," he said.
The race has attracted 11 contestants. Others are Elias Barasa of the People's Empowerment Party, Enock Opara, Julius Masinde and Radon Munoko who are Independents.
Also in the contest are Dr Fredrick Busolo of Vibrant Democracy Party, Aaron Wafula (Republican Liberty Party), Geofrey Barasa (National Ordinary People Empowerment Union) and Robert Sifuna (Roots Party of Kenya).
There are 1,373 polling stations in the county, and the votes will be counted at the county tallying centre at Mabanga.
The County Independent Electoral and Boundaries Commission Returning Officer Grace Rono met contestants with their agents and promised them a fair process.
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