Surely that man's a Democrat? Yes, it's our old pal Zigzag Zell

The question will become a staple of political trivia quizzes. Who gave the keynote speech for different parties at their presidential nominating conventions, both in the same arena? The answer is a 72-year-old Georgian named Zell Miller, known to his Democratic detractors as "Zigzag Zell".

Tonight, Mr Miller, a former governor and now-departing US senator for his state, does the honours for George Bush and Vice-President Dick Cheney at Madison Square Garden, where he delivered the keynote address for Bill Clinton in July 1992. Then he was an moderate southern Democrat, picked by the Clinton/Gore campaign to underline how the party had broken with its liberal past.

Twelve years on, Mr Miller is still, nominally, a Democrat. But he was a Senate sponsor of Mr Bush's controversial tax cuts, and the first senior Democrat to declare his support for John Ashcroft, the most conservative US Attorney General in decades.

He no longer attends the weekly lunches of Senate Democrats, most of whom wish he would make his defection formal. "It's not me who's changed," Mr Miller tells reporters. "It's the Democratic party that's changed."Tonight he commits the ultimate apostasy, by recommending to the nation that it reject John Kerry, a man he described just three years ago as "one of this nation's authentic heroes, one of this party's best-known and greatest leaders and a good friend", and vote for the Republican incumbent.

In 1992, Mr Miller rendered priceless service to Bill Clinton, his then fellow-governor from Arkansas, by helping him win the pivotal Georgia primary, and carry the state in his victory over the first President Bush.

In 2004, the junior Bush is hoping for a reverse Miller effect, that the endorsement of a moderate Democrat may help to persuade coveted undecided centrists and independents to back him. In 1992, the Georgian described Bush senior as weak and out-of-touch.

Today, he calls the party he had supported for half a century an assemblage of factions, for whom "the sun is setting over a waiting grave". This President Bush is "the right man at the right time" to lead the US.

In January, Mr Miller retires from the Senate seat to which he was appointed in 2000. In doing so, he delivers one final stab in the back to Democrats. Almost certainly, the Republicans will capture the seat, making it that much harder for old soulmates to recapture a Senate majority.