BRIAN Cowen today begins the most important week of his political career, aware that Thursday's referendum could define his tenure as Taoiseach.

A victory for the 'no' side will bring an abrupt end to the Taoiseach's honeymoon period, prompting finger-pointing and serious questions about Cowen's handling of the campaign.

Criticism would centre on the failure to secure the crucial endorsement of the Irish Farmers' Association for the treaty until last Tuesday; the squabble with Fine Gael over the intensity of its campaign; and the out-smarting of the main political parties by the relatively inexperienced 'no' side.