The conservative Tea Party movement has won several victories over mainstream United States Republicans in primary contests ahead of mid-term elections in November.
In one of the biggest upsets, a Tea Party-backed candidate beat veteran Congressman Mike Castle for the Senate nomination in Delaware.
Christine O'Donnell was publicly ridiculed and written off by the state's Republican leadership, but survived to score a resounding victory over Congressman Castle.
The BBC reports it is a result that will ring alarm bells for the party leadership.
Delaware is one of a handful of states which Republicans must win if they are to seize control of the Senate.
However, polls suggest Miss O'Donnell will not appeal to the wider electorate.
Meanwhile, Tea Party ally Carl Paladino beat the Republican party's chosen candidate Rick Lazio in the race for the nomination to stand for New York governor.
Republicans are hoping to benefit from anger over the US economy to win both houses of the US Congress in November.
The grassroots Tea Party movement - which favours tax and spending cuts - emerged as a force in American politics only last year.