Campaigning for the May 9 presidential election is heating up, with contenders bolstering attacks against their rivals. This means voters should be attentive and prudent about not letting the vetting process degenerate into negative campaigning.

The most conspicuous recent development in the campaign is that the monthslong solo spurt of Moon Jae-in of the Democratic Party of Korea has all but ended, as the race has quickly shaped into a mostly two-way race between Moon and Ahn Cheol-soo of the People’s Party.

The latest opinion polls show that Ahn has been catching up with Moon, with some even putting Ahn in front. With their support standing between 30 percent and 40 percent each, both are far ahead of the four other candidates.

It is apparent that Ahn, a moderate liberal who was a medical doctor and computer guru, is drawing centrist and conservative voters who had supported former UN Secretary-General Ban Ki-moon and South Chungcheong Province Gov. An Hee-jung.

In other words, many non-liberal voters who don’t support Moon are shifting their support because they believe Ahn has a higher chance of defeating Moon than candidates from conservative parties like Hong Joon-pyo of the Liberty Korea Party and Yoo Seong-min of the Bareun Party and independent candidate Kim Chong-in.

If the past is any guide, the current mold of “two strong candidates and four underdogs” is unlikely to remain as is. For instance, few believe that Sim Sang-jeung of the left-wing Justice Party, which has often teamed up with liberal parties in past major elections, and Kim, who persistently called for a broad alliance of candidates and political groups, will remain in the race until Election Day.

There is also the possibility of Hong and Yoo unifying their candidacy to lure back conservative voters who were estranged by the impeachment of Park Geun-hye. One should not rule out the possibility of alliances even involving Moon and Ahn.

At any rate, electioneering will intensify for the time being, and voters are entitled to vet the candidates -- especially Moon and Ahn -- thoroughly. We are just learning -- through Park -- how damaging and painful it could be if we fail to make a thorough evaluation of those who seek the nation’s highest elected office.

It is wrong in this sense for Moon to keep ignoring allegations about his son and an in-law of late President Roh Moo-hyun. The case of Moon’s son entails allegations he landed a job with the help of his father in 2006, and the other involves an alleged cover-up -- with the involvement of Moon, then as a presidential aide -- of drunken driving by one of Roh’s in-laws in 2003.

Moon, without coming clean about the allegations, has insisted they are part of negative campaigning against him. He said that “that’s enough” with regards to allegations about his son. That is hardly enough, and Moon -- and all other candidates including Ahn -- should be ready to be laid bare before the eyes of voters.

It is not always good for an election to focus on the past, but when it comes to vetting candidates, looking into their past is important for measuring their ethical standards and other individual qualifications. Legitimate questions about them should not be dismissed as negative campaigning.

To be fair, the election is being rushed in the wake of Park’s ouster and voters still have not had enough chance to examine each candidate. In general, primaries within political parties and television debates among contenders did not live up to the public’s expectations that they would help voters decide.

The proposal for a “limitless debate” is therefore a good suggestion. Yoo and his former primary rival, Gyeonggi Province Gov. Nam Kyung-pil, gave a good example when they engaged each other in a “debate without limits to issues.”

Any such duel between major contenders -- like Moon and Ahn -- would greatly help voters determine who is more honest, ethical, competent and visionary. Voters are entitled to get a full grasp of them.