Morales' popularity has been waning and unlike his previous three election victories, opinion polls say this one is likely to go to an unprecedented second round runoff on December 15.
The latest shows Morales with 32 percent of the vote, while his main challenger, former president Carlos Mesa, has 27 percent.
In an eve-of-poll message on Twitter, Morales called on voters in the resource rich Andean state to participate "peacefully and actively" in the elections.
Mesa said he feared the elections would be fraudulent because of Morales' powerful grip on key organs of state, and expressed his concerns in a meeting with observers from the Organization of American States.
Morales said the maturity of Bolivia's electorate would make Sunday's polls a triumph of democracy which would be "an example for delegations and observers who visit us."
Morales, who will turn 60 next week, is already the longest-serving president in Bolivian history, having been at the helm for 13 years.
Mesa has no party of his own but is backed by a minor center-left party. He also has support from a collective of other small parties.
The journalist and historian, who was president from 2003-2005, was a vocal critic of Morales even before announcing his latest bid for office.
Bolivia's 7.3 million electorate will also vote Sunday to renew the 136-seat congress.
Polls open at 8:00 am (1200 GMT) and close at 4:00 pm, according to AFP.
None of the other candidates are expected to come close to challenging the top two, but neither Morales nor Mesa is likely to win outright.