* 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 Santos reaction, quotes from news conference)
By Rosa Tania Valdés and Helen Murphy
HAVANA/BOGOTA, May 16 (Reuters) - Colombia's FARC rebels reached a landmark agreement with the government toward 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, a departure from the forced destruction of coca fields, often by spraying herbicide and with the help of billions of dollars in U.S. antidrug aid.
They also reached a deal on the prevention of drug use and a solution to the production and sale of narcotics.
The FARC, which has turned to coca growing to finance its operations, agreed to help convince farmers to plant other crops. The FARC opposes the chemical destruction of coca, and the government promised to spray fields only as a last resort.
Chief government negotiator Humberto de la Calle called the deal a "landmark" and celebrated what he called a "dream," the possibility that the entire country would "work toward the same goal, in this case, a country without illegal crops and without drug trafficking."
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.
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 voters.
Santos has sought to sell the idea that without his re-election, peace talks could collapse and Colombia would remain at war indefinitely.
"This is the furthest we have ever come on the path to ending our war," Santos said in a televised speech. "Can you imagine a Colombia without coca? This is within reach of our hands if we implement these agreements."
Zuluaga, an ally of influential former President Alvaro Uribe, has 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. (Additional reporting by Luis Jaime Acosta in Bogota, Peter Murphy in Cartagena and Nelson Acosta in Havana; Writing by Daniel Trotta; Editing by W Simon, Bernadette Baum, Andrew Hay, Paul Simao, Jan Paschal and Ken Wills)