* FARC rebels also declare ceasefire to cover presidential election
* Santos gets political boost in his tighter re-election campaign
* Guerrilla war has cost more than 200,000 lives since 1964 (Adds official announcement of drug agreement)
By Rosa Tania Valdés and Helen Murphy
HAVANA/BOGOTA, May 16 (Reuters) - Colombia's FARC rebels reached an agreement with the government on eliminating the illegal drug trade and called a weeklong ceasefire on Friday, giving a political boost to President Juan Manuel Santos in his re-election bid.
The unilateral ceasefire includes the first round of presidential elections on May 25. It was announced after Santos, once a clear favorite, began to falter in public opinion polls.
The center-right president's lead has evaporated with the rise of right-wing rival Oscar Ivan Zuluaga, now tipped to win in two recent surveys if, as expected, the voting goes to a second-round runoff on June 15.
While Santos has staked his political future on the talks, Zuluaga has threatened to end them if he wins.
Friday's agreement puts the two sides one step closer to ending Latin America's longest-running guerrilla war. Rebel and government negotiators meeting in Havana are seeking to end a conflict that has killed more than 200,000 people since 1964.
They agreed to cooperate on eradicating illicit drug cultivation through crop substitution. Though Colombia imports much of the coca leaf used to produce cocaine, the South American country also grows marijuana and poppy, the raw ingredient for heroin.
They also reached a deal on the prevention of drug use and a solution to the production and sale of narcotics.
"The cultivation, production and commercialization of illicit drugs have run through, fed and financed the conflict," the two sides said in a joint statement.
"The end of the conflict should represent an opportunity to find a joint and comprehensive solution to the problem of illegal drugs," it said.
Negotiators have now reached accord on three of the five phases of the talks. Agricultural reform and the rebels' participation in politics were agreed last year, while reparations for war victims and the mechanics of ending the conflict remain the outstanding issues.
Should the two sides reach all five partial agreements, they would engage in a final round reviewing the entire pact.
In its unilateral ceasefire, the FARC announced it would stop all attacks from the start of May 20 to the end of May 28. The National Liberation Army (ELN), another leftist guerrilla group, also said it would call a ceasefire during the period.
Elections have historically been marred by rebel attacks as the guerrillas sought to intimidate and influence voters. The government usually sends thousands of additional troops to patrol around voting time.
Santos has sought to sell the idea that without his re-election, peace talks could collapse and Colombia would remain at war indefinitely. Voters may perceive there is a spirit of cooperation at the negotiating table and that progress is being made under Santos' leadership.
Zuluaga, an ally of former President Alvaro Uribe, has strongly condemned the negotiations and promised to suspend them until the rebels call a definitive ceasefire and accept jail time.
Uribe rose to popularity by taking a hard line against the guerrillas. He remains politically influential. (Additional reporting by Luis Jaime Acosta in Bogota and Nelson Acosta in Havana; Writing by Daniel Trotta; Editing by W Simon, Bernadette Baum, Andrew Hay, Paul Simao and Jan Paschal)