Giving in to mounting pressure, Prime Minister Lee Wan-koo tendered his resignation midnight Monday. President Park Geun-hye is expected to accept the resignation upon her return from a tour of Latin America on Monday.

Given the circumstances, Lee would have had no choice but to step down. When Sung Woan-jong, the late chairman of Keangnam Enterprises, left behind voice recordings and a memo indicating that he gave him illicit money, Lee flatly denied the allegations.

But evidence and testimonies kept emerging to support Sung’s claim that he gave the prime minister 30 million won in illegal election funds during the 2013 parliamentary by-elections.

Lee himself deepened the suspicion by changing his words on his relationship with the late tycoon and the allegation that he personally took the money in his campaign office on the day specified by Sung.

The opposition had enough grounds to initiate a parliamentary motion to remove Lee. With the issue dominating the ongoing by-election campaigns for four parliamentary seats, some members of the ruling party joined the fray against Lee.

Whether Lee did receive illicit money from Sung or not should be determined by the ongoing investigation, but it is shameful that we had to see the No. 2 in the administration make such a disgraceful exit after becoming a prime suspect in a high-profile corruption scandal.

Now that Lee has stepped down as prime minister, the prosecution ought to speed up its investigation into the case, without ― as Park said ― allowing any sanctuary.

The probe into Lee may well set the direction for investigations of other people mentioned by Sung before he committed suicide. Prosecutors will have to be ready to summon Lee as soon as Park formally accepts his resignation.

Deputy Prime Minister Choi Kyung-hwan, No. 3 in the administration, presided over the weekly Cabinet meeting Wednesday as Lee gave up his official duties. This is truly an awkward situation, but it is better than putting up with a prime minister who has already lost his legitimacy for any longer.

What’s important is that the government and the National Assembly do not allow any power vacuum in running the country. This should be a crisis only for the Park administration, not for the nation.