Mithat Sancar, co-leader of the left-wing Peoples' Democratic Party (HDP), parliament's third biggest, made the appeal late on Monday after the six-party opposition bloc announced that Kemal Kilicdaroglu would be its candidate.
"The open support of the HDP would draw (negative) reactions, notably from the IYI Party and its grassroots," he said of the nationalist party, second-biggest in the alliance.
"HDP support is extremely critical," he said, but added it could undermine backing elsewhere.
In campaigning, Erdogan will likely play up alleged links and the HDP's opposition role, analysts say.
The second official, who was not authorised to speak on the record, said some Kurdish voters would not back the opposition alliance due to IYI's involvement.