Selahattin Demirtas, the co-chairman of the Peoples' Democratic Party (HDP), told AFP in an interview there was no contradiction between strongly opposing last week's putsch as well as Erdogan's rule.
The HDP is highly suspicious that a state of emergency declared by the authorities could be used for a wider crackdown beyond the coup plotters, he said.
Demirtas added that the chances of resuming the peace process to end the over three-decade insurgency of the Kurdistan Workers' Party (PKK) are weak so long as its jailed leader Abdullah Ocalan is sidelined.
"Turkey was already afflicted by Erdogan's oppression and the putsch would only have made things worse."
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Demirtas and the HDP turned into a major headache for Erdogan and the ruling Justice and Development Party (AKP), with his natural charisma helping scoop over 13 percent of the vote for the HDP in June 2015 legislative polls.
In a snap re-run of the elections in November the same year, the HDP's share of the vote fell to under 11 percent, with Erdogan targeting the party and accusing it of being the political wing of the PKK.
"But we have not supported Erdogan nor their (the AKP's) policies. In reality, we think that it is the errors made by Erdogan that led to the realisation of this coup."
The coup came almost a year to the day after the breakdown of a truce declared by the PKK that had held for two and a half years. There has been no let-up in the fighting since.
The violence -- which has seen almost 500 members of the Turkish security forces killed in a year -- brought to an end a peace process that had raised hopes for a final deal to end the conflict.