The three key words of Bill Clinton's news conference were those he used to condemn Republican Congressional leaders - "the new isolationists".

It's a phrase the Democrats will be picking up fast.

On Friday, vice-president Al Gore will launch the first television advert of his own run for the White House, and he's promising to make the arms control vote a key campaign issue.

President Clinton: The Senate's vote held "signs of a new isolationism".

Mr Gore will call for the electorate's backing with this pledge: "I believe campaigns should be about the future and there's no more important challenge than stopping the spread of nuclear weapons.

"If elected president, I will send this treaty back to the Senate with your demand that they ratify it."

The Democrats are confident they have public opinion on their side.

Political mileage

The international outcry at US rejection of the treaty has been reflected by newspaper editorials in Washington castigating the "destructive abdication of American leadership".

Most important to aspiring presidential candidates, opinion polls show majority backing for a ban on further nuclear testing.

Most important to Democrats, the man who's charging ahead in the race for the White House, Texas Republican George W. Bush, opposes it.

So Bill Clinton in the White House and Al Gore on the campaign trail intend to make heavy weather of the Republicans' refusal to countenance ratification.

The president warned that the party's hard-line right wingers, while not a majority, "are a very very potent minority". Al Gore described Wednesday's vote as "breathtakingly irresponsible".

Preaching to the converted

Democratic presidential hopeful Bill Bradley is also pro-treaty.

But will the public be listening? The first Gore campaign ads will air largely in Iowa and New Hampshire, the key testing grounds for New Year votes to decide the party nominations for president.

In other words, Mr Gore is aiming to impress an audience of Democratic loyalists with his internationalist fervour, to claim the issue as his own, even though rival Bill Bradley also supports the treaty.

Come the final vote in November 2000, it's hard to see regular voters fired up by the issue of America's global leadership.

If it proves to be a time of international crisis, questions of the superpower's moral obligations will return to the fore.