LA PAZ - Bolivian President Evo Morales led Sunday's election but did not have enough votes to avoid a potentially risky runoff with chief rival Carlos Mesa in December, a preliminary count of nearly 84% of official results by the electoral board showed.Morales, 59, South America's longest-serving leftist leader, won 45% of votes, compared with 38% for the more conservative Mesa, a former Bolivian president, according to the partial count, the Supreme Electoral Tribunal (TSE) said late on Sunday. Morales needed at least 40% of votes with a 10-point lead over the runner-up to win outright. The final winner in the election will govern Bolivia, a landlocked country of 11 million people, from 2020 to 2025.The results indicated that Morales would head to a second-round vote for the first time since sweeping to power in 2006, giving him his weakest mandate and a likely minority of seats in Congress for his party if he manages to defeat Mesa in December.Mesa, who said earlier he did not trust the TSE, celebrated the results amid cheering supporters, saying: "We're in the second round!""Democracy is the most important value for which we are fighting," added Mesa, 66, driving home the theme of his campaign. "We're not going to lose it."A quick count of ballots by two pollsters - Jubileo Foundation and ViaCiencia - showed the race was even tighter, with about 44% for Morales and 39% for Mesa.Votes from rural areas that tend to favor Morales were still coming in, but would not likely give him the boost he needs to avoid a runoff.Morales is running in defiance of term limits and despite a 2016 referendum in which Bolivians voted against allowing him to seek a fourth consecutive term. A local court ruling allowed him to run anyway. As he did in the 2014 election, Morales has promised to retire after the five-year term is over.Chi Hyun Chung, the candidate of the right-wing Christian Democratic party, was running in third place with close to 9% of the vote, indicating his support base will be a key target for Morales and Mesa in the second round.Morales, a former union leader for coca growers, has managed to hang onto power as most other leftist presidents in South America elected in the previous decade have since been succeeded by right-leaning leaders.He has overseen a long stretch of political and economic stability for Bolivia, the continent's poorest country. But support for him has slipped amid slowing economic growth and concerns about government corruption and anti-democratic practices.Whoever wins will likely have to govern without a majority in Congress and with a gloomier economic outlook, as the commodities-fueled boom that drove rapid economic growth in Bolivia in recent years has ended and the country's important natural gas reserves have dwindled.