Primary elections in the United States are almost always deceptive, but they can be dangerous, too. The unexpected victories for Tea Party candidates in this year's last bunch of Republican primaries are a bit of both. Candidates for this populist grassroots movement won, among others, the nomination to run for New York governor and the Senate nomination for the state of Delaware – a state of more importance than it small size suggests because so many major corporations have their headquarters there for tax reasons. Tea Party adherents had earlier won Senate nominations in states as far apart as Florida and Alaska.

To equate primary victories with actual congressional seats or governships, however, would be not only premature, but wrong. In presidential elections it is always said that in the primaries, candidates have to win over the party base – which means campaigning to the right or the left – whereas, with the nomination in the bag, they must do a fast switch and campaign from the centre. Barack Obama is a classic case in point.

The rule in congressional elections is no different. The trick for the nominees is now to campaign in such a way as to appeal to a much broader group of voters. And this is the real test. They have to show that their brand of populism has appeal beyond the many Republicans disillusioned with the lacklustre opposition put up to President Obama by the Republicans currently serving in Congress. In one way, it could be argued, the success of the Tea Party movement reflects the way in which Mr Obama has managed to occupy so much of the centre ground. This has left frustrated Republicans looking for distinctive policies and a distinctive voice, and finding it in Sarah Palin and the Tea Party.

It is far too early to conclude, however, that the mainstream – left or right – has lost the fight. Small government, which is at the heart of the Tea Party movement's philosophy, may be a very American strain of thinking, but in austere economic times it could prove harder to sell. The Republican establishment also fears, not without reason, that Tea Party nominees could scare the voters and so forestall the landslide it is hoping for, come November.

Sarah Palin has down-home qualities that appeal to many Americans, and she should not be underestimated as a political force. The Republican right is yearning for leadership. But neither should she be built into a bogey-woman. She is a long way from being on a fast track to the White House in two years' time.