By Helen Murphy
BOGOTA, Aug 7 (Reuters) - Colombia's President Juan Manuel Santos will be sworn in for his second term on Thursday to face the formidable task of clinching peace with Marxist FARC rebels as a hostile opposition seeks to hobble any accord that passes through Congress.
Santos won re-election in June with the aid of leftist parties that back negotiations to end 50 years of war. In an unusual display of solidarity, they joined forces with the center-right Santos to block former President Alvaro Uribe's right-wing candidate, who had sought to scrap talks.
"Our greatest challenge is peace. We must cultivate peace and we must construct it between us all," said Santos, who will take the oath of office before 12 heads of state.
Although Santos wants his second mandate to move the nation toward a post-conflict era, it will likely be a bitter four years.
Uribe - voted recently to the Senate - has invigorated the opposition, trimming Santos' majority in Congress and probably complicating passage of key reforms and peace-related deals.
Santos, who bet his legacy on talks with the Revolutionary Armed Forces of Colombia (FARC), has used considerable political capital waving the peace flag to win a second term. The launch of the talks in late 2012 enraged Uribe, who heads the biggest opposition party.
A still-popular strongman, Uribe is a bitter rival of Santos and vehemently against the negotiations, preferring to defeat the group on the battlefield instead of allowing it a peaceful entry into society and a possible role in politics.
Uribe will likely be pleased by a high court ruling on Wednesday that bars rebels guilty of atrocious war crimes from public office. It also may complicate the peace discussions underway in Havana since it would prevent many FARC leaders from political participation.
Any agreement with the FARC - which has stepped up attacks in recent weeks - must first go to a referendum and then to Congress for the passage of laws that would deal with the implementation of peace.
"Uribe's opposition to the peace process is a big obstacle, but it's not insurmountable because he doesn't have a majority," said political analyst Marcela Prieto.
"And it's important that there is a balance in Congress to the process because almost half the population has reservations about the talks."
While Santos, a former journalist and scion of one of Colombia's most powerful families, won re-election promising an end to the war, 45 percent of voters backed his rival, Oscar Ivan Zuluaga, and sought an end to the talks.
Uribe's party - the Democratic Center - will likely try to obstruct legislation that would enable FARC rebels to enter the political system without serving considerable jail time.
A recipient of hundreds of millions of dollars in annual U.S. anti-narcotics aid, Colombia has fought the FARC, right-wing paramilitaries and a smaller rebel group, the ELN, since 1964. More than 200,000 people have died and millions have been displaced.
Santos, who turns 63 years old next week, has also started exploratory talks with the National Liberation Army, or ELN, and denies there will be impunity for the rebels.
(Additional reporting by Luis Jaime Acosta; Editing by Lisa Shumaker)