South Koreans voted Tuesday for a new president, with victory widely predicted for a liberal candidate who has pledged to improve ties with North Korea, re-examine a contentious U.S. missile defense shield and push sweeping economic changes.

Conservatives worry that a victory by Moon Jae-in might benefit North Korea and put South Korea at odds with its most important ally, the United States.

But Moon has been the clear favorite as the country's powerful conservative forces struggle to regroup after a huge corruption scandal that led to President Park Geun-hye's removal from office and arrest in March.

"This is the last challenge of my life. I've really done my best so far. I've made enormous preparations. I'm confident. I'll strain every nerve to the last minute to be a president for all the people," Moon, 64, said on the eve of the election.

The final opinion surveys released last week showed Moon, the Democratic Party candidate, had about a 20 percentage point lead over his two main rivals - a centrist and a conservative.

His victory would end a near decade of conservative rule by Park and her predecessor, Lee Myung-bak. When the liberals were last in charge in Seoul, Moon served as chief of staff for then President Roh Moo-hyun. They sought closer ties with North Korea by setting up large-scale aid shipments to the North and by working on now-stalled joint economic projects.

Voting stations opened at 6 a.m. and are to close at 8 p.m. South Korean TV stations plan to release the results of their joint exit polls soon after the vote ends and are expected to predict a winner before midnight.

The winning candidate will be officially sworn in as South Korea's new president after the National Election Commission ends the vote count and confirms the winner on Wednesday. This forgoes the usual two-month transition because Tuesday's vote is a by-election to choose a successor to Park. Her term was originally to end in February 2018. The new leader will still serve out a full, single five-year term.

Park, South Korea's first female president, is currently jailed at a detention facility near Seoul and awaits a criminal trial set to start later this month. She has been indicted on bribery, extortion and other corruption allegations that could theoretically send her to jail for life.

The allegations incensed many in South Korea, with millions taking to the streets and calling for her ouster. Park sympathizers later staged their own rallies. Dozens of high-profile figures, including Park's longtime confidante, Choi Soon-sil, and Samsung's de facto leader, Lee Jae-yong, have been indicted along with Park.

The drama gave Moon, who lost the 2012 election to Park by a million votes, a boost in his push to re-establish liberal rule.

Frequently appearing at anti-Park rallies, Moon called for her ouster and reform measures to clean up social inequalities, excessive presidential power and corrupt ties between politicians and business leaders. Many of these legacies dated back to when South Korea was ruled by Park's dictator father, Park Chung-hee, a deeply divisive figure whose 18-year rule was marked by both rapid economic rise and severe civil rights abuse.

As a former pro-democracy student activist, Moon was jailed for months in the 1970s while protesting against the senior Park. He later worked as a human rights lawyer and chief of staff for Roh, who governed in 2003-2008.

Moon has called Park Geun-hye's hard-line North Korea policy a failure. If elected he says he'll employ both pressure and dialogue to persuade the North to abandon its nuclear ambitions. He also advocates building up a more assertive South Korea and is critical of Park's decision to allow Washington to install the Terminal High-Altitude Area Defense (THAAD) advanced anti-missile system in the South. The system has irked Beijing, Seoul's largest trading partner.

Following a standoff between Trump and North Korean dictator Kim Jong Un over Kim's reported nuclear test plans, Moon has talked more about bolstering national defense and said THAAD deployment is inevitable if North Korea provokes. Critics say Moon was looking to woo conservative voters.

"I'll take charge so you won't have to worry about security, national defense and peace," Moon said Monday in a message addressed to senior citizens, many of them conservative voters who oppose Moon because of what they see as a soft North Korea policy.

Even if he becomes leader, many analysts say Moon won't likely pursue drastic rapprochement policies because North Korea's nuclear program has achieved too much progress since he was in the Roh government a decade ago. Foreign experts say it may take only several years for North Korea to develop nuclear-armed missiles that can reach the U.S. mainland. The country may already have shorter-range nuclear missiles that can strike South Korea and Japan.

Moon's nearest rivals are Ahn Cheol-soo, a centrist who has shown a more conservative stance on North Korea, and Hong Joon-pyo, a member of Park's embattled party who has called for the reintroduction of U.S. tactical nuclear weapons in South Korea.