The controversy centres on accusations, now under investigation by a federal grand jury in Florida, that Ron Brown, Commerce Secretary and former Democratic party chairman, agreed to accept dollars 700,000 (pounds 460,000) channelled through exiled Vietnamese businessmen in return for helping to lift the economic sanctions.
Thus far, the case is anything but proven. No directly incriminating evidence has been produced, and Mr Brown has dismissed the allegations as 'preposterous' and 'absurd'. The President himself has gone out of his way to display his support for a man who played an important part in his election campaign.
But there is no mistaking the nervousness in the White House, as further details have emerged in the past few days. Mr Brown's main accuser, a bankrupt Miami businessman called Ly Tranh Binh, has passed an FBI lie-detector test, while the Bureau, according to press reports this week, is on the track of mysterious bank accounts in Singapore. Moreover, dents in the Commerce Secretary's original story have emerged.
Reversing earlier denials, Mr Brown now acknowledges he did meet a key middleman in the affair three times, most recently last February, though he insists the contacts were utterly innocuous. Nor is he helped by his background as a high-powered Washington lobbyist, with clients ranging from Japanese multinationals to the 'Baby Doc' Duvalier regime in Haiti. These activities stirred considerable criticism before his confirmation by the Senate last January.
Whatever else, the affair is heaven-sent for conservative Republicans and Vietnam veterans' groups adamantly opposed to normalisation or relations with Hanoi until the fate of US servicemen still unaccounted for is resolved.
Much is riding on the outcome for both countries. Vietnam wants to attract US investment to speed the modernisation of its economy, while US companies are no less anxious to enter a huge potential market which is already being tapped by their Japanese and European competitors.
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