The seat is currently held by Rajya Sabha deputy chairman P J Kurian of the Congress who will retire on July 1.
The KC(M) had walked out of the Congress-led UDF in August 2016, ending over three-decade-long association alleging it was being "insulted" by the party leading the alliance.
Relations between the two parties had soured over the bar bribery scam, and the rift widened after Congress leaders Oommen Chandy and Ramesh Chennithala attended the betrothal ceremony of the daughter of controversial hotelier Biju Ramesh and the son of Congress leader and former minister Adoor Prakash.
Mani also attended a meeting of the coordination committee of the UDF today where he was received by the Leader of the Opposition in the state Assembly Ramesh Chennithala, AICC general secretary Oommen Chandy, and IUML general secretary P K Kunhalikutty, an MP.
The Congress's decision to spare the Rajya Sabha seat for KC(M) candidate has not gone down well with a section of the party.
The state Congress's former president V M Sudheeran did not attend the UDF meeting, insisting the development would weaken the main opposition party and benefit the BJP.
The decision, he said, lacked transparency and party workers were left disappointed.
Chennithala, Chandy and KPCC president M M Hassan had said the decision to allot the RS seat was taken to strengthen the UDF as part of the party's resolve to bring together democratic and secular forces to take on the BJP in the Lok Sabha polls.
"The RS seat was given to KC(M) as a special case," Chandy said.
Reacting sharply to the development, Sudheeran said, "The party should not have surrendered its pride to KC(M) to bring it back into the front."
Echoing similar sentiments, Kurian had said no discussion was held in the party or its election committee. "This is a surrender of the Congress," he told reporters in New Delhi last night.
After it snapped ties with the Congress nearly two years ago, the KC(M) maintained it was keeping equidistance from the UDF, the CPI(M)-led Left Democratic Front, as also the BJP, which is trying to create a sustainable political space for itself in Kerala.
The KC(M), however, extended support to the UDF candidates in all the three by-polls held in the state since it left the alliance.
Six Congress MLAs also criticised the party's decision to allot the RS seat to KC(M).
Three Rajya Sabha seats from Kerala are falling vacant this month. Of these, the ruling LDF is in a position to win two and the UDF one. It was widely expected that the Congress will put up its own candidate for the seat.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)