Seven trade unions on Friday endorsed Nasa presidential flag bearer Raila Odinga's bid.
Under the Trade Union Congress, the workers said President Uhuru Kenyatta's Jubilee government has failed them since 2013.
The Unions Are
Others include Kenya Medical Practitioners and Dentists Union and the Kenya National Union of Nurses.
The groups said they will vote Raila on Tuesday. The Opposition chief is facing off Uhuru of Jubilee Party in a race that has eight contestants.
"Jubilee government has perfected the act of criminalizing every worker in this country. They have taken teachers, doctors, and nurses to court for demanding their rights," Wilson Sossion, TUC secretary general, said.
Sossion, who is also Knut secretary general, said NASA has taken care of all workers' interest in the country.
"Doctors were on strike for 100 days. Jubilee did not care. As we are speaking nurses strike is 61 days."
"As workers of this country, we cannot continue to suffer under Jubilee," Sossion said.
"We have the numbers and on Tuesday we will vote for NASA."
However, he failed to say what will happen if Jubilee retains the presidency after the Tuesday vote.
The union official spoke at a press conference hosted at Mombasa governor Hassan Joho's office. He was flanked by other union officials.
Raila, who was present, said he is confident NASA will form the next government.
He said trade unions signing an agreement with political parties is not something new in the world.
"It has been done somewhere else in the developed countries. Our engagement with trade unions started one year ago," Raila said.
"We have come a long way. These unions engaged both political divides; NASA and Jubilee, but the latter turned a deaf ear on them."
"In NASA, we are committed to accommodating them," he said.
Raila spoke ahead of the final NASA Mombasa rally which will be held at Mama Ngina grounds.