When a candidate for national political office makes a charge that someone has offered him something of value to influence his campaign, that has the appearance of being a potential criminal offense.

In a word, it could be bribery.

That’s true even if the candidate makes the charge against members of his own party, and party leaders say there’s no substance to it because – get this – they investigated it themselves and found it baseless.

That bizarre situation actually happened in the Democratic primary for U.S. Senate in Pennsylvania.

U.S. Rep. Joe Sestak said he was offered a federal job by an unidentified member of the Obama administration if he would drop his plan to challenge Sen. Arlen Specter for the party’s nomination.

Specter had received an endorsement from President Obama after the former GOP senator, facing a strong challenge from longtime rival Pat Toomey, switched parties last year to run as a Democrat.

Since the primary, which Sestak easily won, he has repeated the job-offer charge on national television, but so far has refused to identify the person who made the offer – casting considerable doubt on the accuracy of his claim.

Nevertheless, he made the charge and has repeated it, and the potential offense is critical enough that administration stonewalling only adds credence to the allegation. 

Sestak did say in response to an interviewer’s question that he would cooperate with a Justice Department probe of the claim, but so far the administration has said it has no plans to pursue the issue.

That’s not the right answer. Brushing this aside only makes it a permanent stain on the reputation of a president whose claims to political purity were a foundation of his campaign for office.