The popularity of the Tea Party appears to be on the wane after more than a year of upsetting mainstream politicians, according to a Washington Post/ABC poll published today.

The poll, the first major one to show support for the Tea Party going into reverse, reported that support for the anti-establishment, rightwing grassroots movement has dropped. The percentage of Americans holding an unfavourable view of the movement jumped by 11 points to 50%.

But the poll may turn out to be premature, as the anti-incumbency mood across the US shows little sign of abating. The same poll showed that support for Congress was at an all-time low at 29%.

The Tea Party movement, which began last year as a reaction to the high level of public spending and hostility towards Barack Obama, has over the last six months taken a series of political scalps, ousting centrist Republican candidates in a series of primaries. One of its main champions, Rand Paul, won in Kentucky last month.

The movement has already forced the Republican party to move its policies to the right, even though its campaign strategists tell the leadership it has to shift into the centre to win back ground.

Voters went to cast ballots in primaries in 11 states today ahead of the November mid-term elections, but few of the primaries involve high-profile Tea Party candidates, making it harder to judge whether it has stalled.

In South Carolina Nikki Haley emerged as a frontrunner in the Republican primary for governorship after being endorsed by Sarah Palin, a Tea Party favourite. She has increased her popularity in state polls in spite of allegations about her sexual relations. In another governorship race, in Iowa, Palin endorsed the former governor Terry Branstad.

In Nevada, where Republicans have been queuing up to take on the Democratic leader in the Senate, Harry Reid, the Tea Party favourite is Sharron Angle.

The anti-incumbency feeling is also taking its toll on the Democratic side. Blanche Lincoln, one of the senators from Arkansas, failed to win outright last month and today she faced a run-off.

Trade unions and some liberals have been campaigning against her, throwing money and resources behind her rival Bill Halter. Part of the hostility is because they see her as not having been enthusiastic enough in support of health care reform.

The anti-incumbency mood has already claimed two senators, the Democrat Arlen Specter in Pennsylvania and the Republican Bob Bennett in Utah.