ANKARA, Turkey — Out of the repressive climate of the last 18 months since a bloody coup attempt in Turkey, the most fearless challenger to President Recep Tayyip Erdogan — and potentially his most serious rival in future elections — has been Meral Aksener.

A former interior minister and seasoned right-wing politician, Ms. Aksener, 61, is credited with gathering much of the national support for the “No” campaign in an April referendum that nearly defeated Mr. Erdogan’s attempt to vastly expand his powers.

Now she plans to build on that support to challenge him for the presidency. She predicts that Mr. Erdogan will call early presidential elections for July 15.

Expelled from the Nationalist Movement Party in September 2016 after challenging its leader over a growing alliance with Mr. Erdogan, she founded the Iyi Party, or Good Party, a year later in October. She is already campaigning around the country.