Like everybody else, National Assembly Speaker Park Hee-tae deserves to be presumed innocent until proven guilty. This basic right should hold even though a prosecutor raided the home of one of his erstwhile aides for search and seizure as part of a criminal investigation into an allegation that he bought votes for his 2008 election to the chairmanship of the ruling Grand National Party.

But it does not mean that he should allow himself to retain the top post of the legislature. Instead, he should immediately cut short his 11-day tour overseas, return home, resign as speaker and subject himself to a criminal investigation. As a prosecutor-turned veteran politician, he must do this.

Park denied his involvement in the money-for-votes scandal and embarked on the tour on Jan. 8 ― the day Rep. Koh Seung-duk of the Grand National Party presented himself to the prosecutors investigating his earlier allegation that he received an envelope containing 3 million won in cash from one of Park’s aides and sent the money back. Park’s office said that his tour of diplomacy, scheduled three months ago, could not be arbitrarily canceled.

That was offered as an explanation. But it sounded like a lame excuse, given that parliamentary diplomacy is often a cover for lawmakers’ junkets. Diplomacy of substance is conducted by professional diplomats or administration officials dealing with issues of concern to foreign countries. Moreover, Park’s hosts, aware that he is involved in a scandal back home, would most likely feel relieved if they were told his forthcoming visit was canceled.

Shortly before the ruling party held a national convention to pick a new leader in July 2008, an aide to Park visited Koh’s office during his absence, allegedly carrying a bag containing “so many yellow envelopes.” Koh quotes one of his secretaries as saying that the aide took one envelope out of the bag and gave it to her and that it contained 3 million won in cash.

Still worse, the cash contained in the envelopes may not have been all the money that was spent for his election. Party members acknowledge that additional money must have been used to bus supportive deputies from provinces to the national convention in Seoul and entertain them.

If so, where did the large sum of money come from? Some in the party say it might not have come from his pocket. Instead, they suspect that it might be part of the money left over from the party’s 2007 presidential election campaign fund or that it might have been collected from powerful members of a party faction close to President Lee Myung-bak, who could not stand the idea of Rep. Chung Mong-jun, a potentially hostile outsider, being elected chairman.

Others say that, given his background as a prosecutor, he is not the type of politician to sully his hands by directly involving himself in an attempt to buy votes. Indeed, he lays claim to innocence, saying he was not aware of any such attempt.

Against this backdrop, a member of the party’s emergency governing council said that Park “sustained irreparable damage, both politically and morally, though he might be absolved of legal accountability.” That, the emergency council member said, is the reason why he should resign as speaker.

The council member is not alone in demanding his resignation. Others in the party are calling on him to return home and tender his resignation immediately, instead of continuing his tour and, by doing so, smearing Korea’s image in the countries he is visiting. Moreover, the opposition Democratic Unified Party is moving to pass a resolution calling for his resignation.

Whether he quits or not, it looks to be a matter of time until he is summoned by the prosecutors’ office. He is well advised to look back on his 24 years as a career politician and ponder the choices available to him before making a final decision on what action to take. But he should be reminded that if he decided to hold onto his post to the last moment, he would be the first speaker to undergo a criminal inspection since the legislature was inaugurated in 1948.