The 2008 Democratic candidates are beholden to a “hyper-partisan, politically paranoid” liberal base that could endanger the final nominee’s chances of winning next year’s presidential election, Joe Lieberman, the former vice-presidential Democratic candidate, said on Thursday.

In his most outspoken attack on fellow Democrats since he was unsuccessfully challenged last year by Ned Lamont, a liberal Democrat, for his Senate seat in Connecticut, Mr Lieberman on Thursday said he may not vote for the Democratic presidential nominee next year.