Luis Arce, the candidate for Evo Morales's Movement for Socialism party, said he would return to Bolivia Tuesday from exile in Mexico to begin campaigning for May 3 presidential election.
"Tomorrow I'm going to be in Bolivia," Arce told a news conference during a stopover in the Argentine capital Buenos Aires, where Morales -- who appeared alongside him -- is based.
Arce, a 56-year-old former economy minister, joined growing criticism of interim President Jeanine Azez who announced on Friday that she would run in the elections.
"The fact that the de facto president is running raises huge questions about the transparency of the electoral process," Arce told reporters.
Anez assumed the presidency on November 12, two days after Morales resigned following three weeks of sometimes violent protests against his controversial re-election in a poll the Organization of American States said was rigged.
Morales initially fled to Mexico, where he was given asylum, before moving to Argentina in December.
Former presidents Carlos Mesa and Jorge Quiroga have also joined the presidential race along with regional leader Luis Fernando Camacho, a key player in the protests that led to Morales's resignation.
The Movement for Socialism has been leading in the polls, with 26 percent of voter preferences compared to just 12 percent for Anez, who is running fourth.
"In all the polls we are first," Morales tweeted in reaction. "We are ready to beat the coup and regain the homeland."