It was to be the first stirring of a liberal-left realignment in American politics.

The Republican reign had run its course. You might think, if you're against the Iraq war and want the Democrats to win the presidency in 2008, that this is a good thing. You'd be wrong.

We woke up on Thursday morning to find al-Qaeda Islamofascists alive, well and trying to blow up UKoriginating aircraft over five American cities.

Senator Lieberman, stung by his defeat, said something to the New York Times that I was going to predict someone would say. He beat me to it. "If we just pick up like Ned Lamont wants us to do, get out [of Iraq] by a date certain, it will be taken as a tremendous victory by the same people who wanted to blow up these planes in this plot hatched in England, " Lieberman said. "It will strengthen them and they will strike again."

First let's just recap what happened on Tuesday. Joe Lieberman is a centrist member of the Democratic Party and has been for 30odd years. He was a vocal opponent of the Vietnam War. He supports a woman's right to obtain an abortion.

He's for gun control and against tax cuts. In other words, he's got form and is in tune with his party's base on almost every issue. So why did he lose?

The tipping point in American public opinion about Iraq happened some time last December. It was when we realised how badly we'd messed up. The question since has been how quickly we could get out without making things worse.

Energised that they'd won the argument, opponents of the war . . . led by bloggeractivists now called the 'netroots' (rather than grassroots) . . . did not rest on their laurels. They saw Lieberman as a target, because he'd voted for the war and refused to recant when fortunes changed.

This was not a general election, however, and the rookie mistake is to take it as an indicator of a shift in public sentiment. Only registered Democratic party members could vote.

National attention was focused on the race. The netroots had staked their newly-acquired rep on beating Lieberman. They succeeded.

They hope, in the words of the Daily Kos blog, to crash the party of Washington insiders and take over the Democratic party.

Republicans are desperate for them to succeed. Here's why.

In 1972, running against a president only slightly less popular than this one, Democrats nominated antiwar lightweight George McGovern. For the second election in a row the party had been seized by its emotionally-charged fringe incensed about the centrists' original support for a war that had gone horribly wrong.

The centre then deserted the Democrats, making them the party least likely to win the presidency and triggering the slow collapse of their control of Congress.

Voters want leaders to fix mistakes, not preen their moral feathers. And voters sense . . . seemingly confirmed by the bumblings of Jimmy Carter over Iran and Bill Clinton's failure to stamp out al-Qaeda . . . that Democrats were less able, or less willing, to keep them safe in a world that intends them harm.

Netroots Democrats will now be charged with planning to act irresponsibly, pull out of Iraq immediately and dismiss the larger terrorist threat. This charge will enrage them, and could set up the dynamic that actually keeps the Democrats out of power. The better line for anti-war activists is to make a distinction: Iraq was a costly mistake and those who made it should be punished. But they need to show they understand there is a wider struggle that the US and the west have no choice but to wage. It isn't the fantasy of Dick Cheney and Donald Rumsfeld. There really are people who want to kill us.

We need to elect people better able to stop them.