Former UN Secretary-General Ban Ki-moon arrives in Seoul on Thursday.

Returning home as a major presidential contender, he will likely be given a hero’s welcome by his supporters and be met with disdain from his critics.

Ban’s homecoming after 10 years as chief of the world body signals the official start of his pursuit of presidential power, which could put the presidential campaign -- already showing signs of premature overheating -- into high gear.

The main opposition Democratic Party of Korea, whose former leader Moon Jae-in is running ahead of the pack of potential candidates, has already set a timeline under which it will nominate its candidate in March.

That plan is based on the assumption that the Constitutional Court will uphold the parliamentary impeachment of President Park Geun-hye, which would give the nation two months to hold an election.

It also reflects the party’s confidence -- arrogance in the eyes of critics -- that its victory over the conservatives can be taken for granted as they have been devastated by the raging Choi Soon-sil scandal.

The escalating fights among the party’s hopefuls, including Seongnam Mayor Lee Jae-myung, South Chungcheong Gov. Ahn Hee-jung and Seoul Mayor Park Won-soon, show that they believe whoever wins the Democratic Party ticket gets the Blue House.

At the same time, the Democratic Party and its candidates are united in finding a common enemy in Ban, who poses the biggest threat to them.

Democratic Party Floor Leader Woo Sang-ho criticized Ban for supporting the 2015 Korea-Japan agreement on Japan’s wartime sex slavery, as the issue has become contentious again in the wake of Japanese retaliation against a statue in Busan symbolizing “comfort women.”

The party’s spokesperson also condemned Ban’s performance as the UN chief, arguing that he failed to address the refugee problem, the war in Syria, epidemics and North Korea’s nuclear threat. The opposition also raised its voice against a corruption scandal involving Ban’s brother and nephew.

These all-out attacks on Ban reflect the party’s wariness of his potential candidacy.

Ban recorded 21.5 percent in approval ratings, compared to Moon’s 26.8 percent, in the latest poll. He is being wooed by political groups that are opposed to Moon and the Democratic Party.

The ruling Saenuri Party, despite -- or because of -- its fall caused by the Choi scandal, still hopes to enlist Ban. So does the Barun Party, which broke away from Saenuri, and some senior members of the minor opposition People’s Party, including former Floor Leader Park Jie-won.

Moreover, some presidential hopefuls, such as Sohn Hak-kyu, have suggested a wider “third party” alliance, calling for a coalition of political forces apart from the supporters of Moon and Park.

Ban has not yet given any strong clue as to what path he would take. He only said he would try to exchange views with “as broad a number of people and groups as possible.”

What is certain is that Ban will be the largest variant and whatever path Ban takes will shake the presidential election and politics as a whole.

Presidential elections are usually contaminated by extreme conflicts, between candidates, regions, generations and ideologies.

The unfolding Choi scandal, the forthcoming Constitutional Court’s decision on Park’s fate and the possibility of an earlier-than-planned election may combine to exacerbate the situation.

Ban has emerged as a major presidential candidate mainly due to the public antipathy toward politicians and the establishment, which further deepened in the wake of the Choi scandal.

His strengths and competitive edge should stem from where he differs from old-time politicians. People will closely follow what he does and says in the days, weeks and months to come.