Leading article: In deepest Connecticut, the plates are shifting

It may seem a stretch from the present mayhem in the Middle East to a primary election battle in leafy, tranquil Connecticut, by many measures the wealthiest single state in the most affluent country in the world. But the connection is not only real. It is of vast potential import for US politics and for the future of American foreign policy - and thus for the future of us all.

The contest pits Joseph Lieberman, one of Connecticut's two Democratic Senators and the vice-presidential running mate of Al Gore in 2000, against a hitherto little-known businessman, Ned Lamont, for the right to carry the party's banner against the Republican candidate in November's congressional elections.

A couple of years ago, the thought of a challenge to Mr Lieberman from within his own party would have been a stretch in its own right. He was, after all, one of the best-known Democrats in the country, a widely respected politician who in 2004 made his own run for the White House. Even six months ago, he was leading his opponent by more than 30 per cent. Today, however, the gap has narrowed to single digits, and a stunning political upset next month is very much on the cards. The reason may be summed up in one word: Iraq.

Mr Lieberman was a vigorous supporter of the war from the outset, and remains one today, even though its cost is $6bn a month and American casualties exceed 2,550, while the country on which the US has expended so much blood and treasure slides towards civil war. But this unwavering backing for President Bush has increasingly isolated him in his home state, where opposition to the war is running at almost 70 per cent. It has also earned him the fury of grass-roots Democratic activists, convinced that the Washington-based leadership is losing touch with the party's core supporters and core values.

With increasing confidence and skill, Mr Lamont is exploiting these weaknesses. He insists his campaign is about other issues as well, such as the loss of American jobs overseas, and the inability of the US to offer universal health coverage to its citizens - failures, he says, in which Mr Lieberman and other centrist Democrats have been complicit.

But his selling point is his opposition to the war. Had all been sweetness and light in Iraq, Mr Lamont would never have entered the fray. Now that he has, however, the foreign policy rift in the Democratic party has been laid bare for all to see. And among the most interested observers has been one Karl Rove, master of winning elections for Republicans in general and for George W Bush in particular.

A couple of months ago, the Democrats looked a sure bet to regain either the Senate or the House of Representatives in November, paving the way for the recapture of the presidency in 2008. No longer. There is a palpable sense that the Republicans are making a modest recovery - thanks in part to the Democrats' disarray on Iraq, underlined by the primary in Connecticut.

Mr Lieberman's Democratic colleagues in the Senate have split three ways. Some refuse to back him even in the primary. Others support him now, but will not if he loses and (as he plans) runs as an independent. A third faction vows to stay with the Senator through thick and thin, be he Democrat or independent.

Given the Iraq débâcle, national security should have been the Republicans' Achilles heel in November. With typical counter-intuitive brilliance, Mr Rove has now chosen to make it the centrepiece of the autumn campaign. The strategy worked in 2002, and carried Mr Bush to victory in 2004. Will it work again in November 2006, and help another Republican into the White House in 2008?