Costa Rica's center-left presidential candidate Luis Guillermo Solis is expected to cruise to victory in the run-off election after his ruling party rival quit campaigning in a bizarre twist last month. REUTERS/Juan Carlos Ulate/Files

San Jose - Opposition candidate Luis Guillermo Solis is such a shoo-in for Costa Rica's presidential run-off election Sunday that his only rival for the job threw in the towel a month ago.

With an unbeatable lead in the polls, Solis is now campaigning to make sure the 3.1 million voters in Latin America's oldest democracy don't stay home.

“Go out and vote,” he says over and over again in campaign rallies.

As the candidate of the centrist Citizen Action Party (PAC), the 55-year-old was the unexpected winner of the first round of the elections February 2, coming from last place in the polls to edge out Johnny Araya, the candidate of the ruling National Liberation Party (PLN).

Though still short of the 40 percent needed to take the presidency outright, Solis' popularity continued to surge and by early March a poll showed him with nearly a 45 percent lead over Araya.

Humiliated, Araya dropped out on March 5.

His name remains on the ballot, however, because the constitution prevents him from formally withdrawing. The PLN, meanwhile, continues to appeal for Araya votes “for dignity's sake.”

With his path wide open, Solis is set to become the first third-party candidate to win the Costa Rican presidency in 50 years.

Power here has traditionally gone back and forth between the social democratic PLN and the Social Christian Unity Party (PUSC).

“The time has come for change,” Solis, a historian and academic who left the PLN in 2005 because of its embrace of neo-liberal policies, said in his closing campaign speech.

He promises to fight corruption, redistribute wealth and revive the economy - a message that has energized his supporters.

Faustino Desinach, a 54-year-old photographer, says he hasn't voted in 30 years but will on Sunday “because I believe Luis Guillermo can straighten the boat.”

Corruption scandals and rising debts and deficits under outgoing President Laura Chinchilla, Costa Rica's first female president, has weakened the old two-party system.

But if Solis is indeed elected Sunday to a four-year term, he will still have to deal with a politically diverse Congress in which his PAC party is second in strength behind the PLN.

“Solis has raised many expectations. If he succeeds in adopting a different style of leadership, with a greater openness to dialogue, he will have the political space to confront complicated problems,” said sociologist Manuel Rojas.

Political analyst Jaime Ordonez said restoring confidence and uniting the country “will require intelligence, character and great serenity.”

“We have reached a breaking point. Our entire state is designed for a two-party Costa Rica that no longer exists,” he said.