The electoral backlash from US conservatives has intensified, as Tea Party candidates once again confounded commentators by defeating mainstream Republican candidates in the latest round of congressional primaries.

Undoubtedly the biggest upset of the night came from the traditionally-Democrat state of Delaware, where Sarah Palin-endorsed Tea Party candidate Christine O'Donnell defeated long-serving Congressman and former governor Mike Castle to be the Republican candidate for Vice President Joe Biden's former Senate seat.

O'Donnell, who is pro-gun, anti-abortion, fiscally conservative and believes masturbation is a sin, defeated Castle with 53 per cent of the vote. Given that just a week ago O'Donnell was engaged in bitter in-fighting with some of Delaware's elected Republican officials and had been termed "unelectable" by some fellow Republicans, her victory in last night's primary is not only a blow to the GOP, which committed significant resources to the fight in Delaware in an attempt to prevent a repeat of the upset in Alaska, but also adds yet another dimension of unpredictability to the final outcome of November's midterm elections.

The majority of states have now held their primaries. Eight mainstream Republican candidates have been defeated by Tea Party or otherwise right-wing challengers, a number which could rise to nine pending the outcome of the recount in New Hampshire.

The consensus, especially from the left-leaning commentariat, seems to be that while these primary victories for anti-incumbent candidates demonstrate the power and reach of this new right-wing movement, it is very unlikely that any of these challengers will be victorious in the midterms themselves.

In an election season that had otherwise long been considered to be potentially disastrous for the Democrats, with the possibility that the Republicans could regain control of both the House and the Senate, these ultra-conservative candidates represent an opportunity to claw back some momentum in advance of polling day. For instance, Delaware, previously considered to be a serious prospect for the Republicans, is now much more likely to be a hold for the Democrats, especially if their candidate, Chris Coons, is able to capitalise on Christine O'Donnell's unpopularity with a significant faction of Delaware Republicans.

Just as a footnote, it's worth noting that the electoral fortunes of some of these insurgent candidates could have a knock-on effect for Sarah Palin's presidential hopes. Palin notably endorsed Kelly Ayotte in New Hampshire and Carly Fiorina in California, as well as O'Donnell in Delaware. As she is apparently already moving into position for a 2012 campaign, embarrassing defeats for candidates Palin has personally endorsed and campaigned for could dent her appeal to Republicans beyond the confines of the Tea Party movement.