Three opposition parties in Congo-Brazzaville have joined forces ahead of the next elections, for which veteran hardline leader Denis Sassou Nguesso is a potential candidate.

The "Alliance for Democratic Change in 2026," unveiled on Thursday, brings together three parties that have grassroots support but no legislative seats.

Its members are the Rally for Democracy and Development (RDD) set up by former Marxist-Leninist president Jacques Joachim Yhombi-Opango, the Republicans' Movement (MR) and the People's Party (PAPE).

"There are other (parties) which will be coming along," said Jean-Jacques Serge Yhombi-Opango, the RDD's vice president and son of the party's founder, who died from Covid in 2020.

"There has to be political and democratic change in 2026. We can't let things carry on like this, because the country's going down the drain."

The Republic of Congo, also known as Congo-Brazzaville to distinguish it from its giant neighbour the Democratic Republic of Congo, is a major oil exporter but suffers from widespread poverty and an entrenched reputation for graft.

The former French colony ranks 164th out of 180 in Transparency International's 2022 Corruption Perceptions Index.

At 80, Sassou Nguesso has notched up a total of nearly 40 years in power.

Many Congolese think it likely he will bid for a fifth term in 2026 despite his advanced age.

Past president elections have been stained by allegations of voting fraud, violence and prosecution of candidates.

The new alliance said it would focus on trying to overhaul the country's electoral supervisors.

"The bodies which plan and carry out all pre-electoral and electoral operations are monopolised by a single party, the ruling party. The result is that the rules are skewed from the start," said Jean-Jacques Agnangoye of PAPE.