A local observer group, the Center for Monitoring Election Violence, said this week there had been "unparalleled misuse of state resources and media" by Rajapaksa's party and that police inaction had given free rein to election-related violence.
Rajapaksa won around 58 percent of the vote in the 2010 election, surfing a wave of popularity that sprang from the defeat of Tamil Tiger separatists who had waged a crippling war against the government for 26 years.
The economy has flourished since then and big infrastructure projects such as the country's first expressway have sprung up.
Many voters, especially Sinhalese Buddhists who represent 70 percent of the population, are diehard Rajapaksa supporters. Sandamalee de Fonseka, a 39-year-old mother of two in Colombo said he won the country's war on terrorism and was winning the war against poverty.
"People's quality of life has gone up," she said.
But many complain of high living costs - even though official data show inflation below 5 percent - rampant corruption and an authoritarian style that has concentrated power in the hands of the president's family.
Ethnic Tamils in the country's north and east feel the president has abandoned them since the war while Muslims, the country's third-largest minority, feel under threat from a growing strain of militant Buddhism that was behind a spasm of violence last year.
On foreign policy, Rajapaksa has cold-shouldered neighbouring India. He has also fallen out with Western countries that want an international probe into possible war crimes and criticise his record on human rights, turning instead to China as a strategic and investment partner.
Rajiva Wijesinha, one of the first lawmakers to cross to the opposition in November, said Rajapaksa appeared to have lost touch and had allowed the country's budget and economic planning to go "completely haywire".
"I think he needs to be defeated," Wijesinha told Reuters.
Sirisena, who would lead a potentially fractious coalition of ethnic, religious, Marxist and centre-right parties if he wins, has pledged to abolish the executive presidency that gave Rajapaksa unprecedented power and hold a fresh parliamentary election within 100 days.
He has also promised a crackdown on corruption, which Wijesinha said would include investigations into big infrastructure projects such as a $1.5 billion deal with China Communications Construction for a port city.
"I don't think there will be a witch hunt," he said. "We are not in this for punishing people, but we do want the Sri Lankan people to get their money back."