Jon Stainbrook doesn't try to keep the excitement out of his voice. "In 2006, in 2008, if you had an R by your name, you were gonna lose," he says. "Now if there's an R by your name you win." In next week's midterm elections, he says, the Republicans are "gonna win and gonna win big. It'll be the biggest victory we've had in years."

All this, understandably, has Stainbrook rather excited. He's chair of the Republican Party in Lucas County, Ohio, one of the most solidly Democratic areas in the Midwest. But Ohio as a whole is a depressed, post-industrial wasteland that tends to be a bellwether come election time: as goes Ohio, so goes America. And this year, the state looks set to pick a Republican governor, a Republican senator and a swath of Republic congressman.

"I never thought that people would turn on Obama this quickly," says Stainbrook, with undisguised glee.

Yet if there's a downside for the Republicans, it is contained within the very movement that has done so much to energise its voter base this year. The Tea Party [2] has been festooned with media coverage and had a number of notable victories in helping right-wing Republicans beat more moderate candidates in the party's primaries last summer. More importantly, for the first time in half a decade, it has got the party's conservative base excited about being conservative again.

But while the Tea Party may have helped the Republicans out this year, there are reasons to think it could be a liability in future elections.

Stream of anger

Problem number one is that the Tea Partiers are, not to put too fine a point on it, nuts. Horror stories about the movement's preferred candidates abound. Christine O'Donnell [3], the conservative running for the Delaware Senate seat, won unflattering headlines when she flatly denied that the US constitution had anything to say about the separation of church and state.

The activists are no better. Gloria Johnson, chair of the Democratic Party in Knox County, Tennessee, says her local Tea Partiers "couldn't organise their way out of a paper bag". Those turning up to a meeting in her district found that the parking garage was closed. "They couldn't face the idea of on-street parking. So they cancelled the meeting."

This is not the stuff that revolutions are made of.

A bigger problem with the Tea Party, though, is that, by picking hard-right candidates, it may be making the Republican Party less attractive to mainstream voters. Many Tea Partiers think they represent a stream of anger that runs through the entire US population. Actually, polls have found that – surprise, surprise – they are far more conservative than most of their countrymen.

This means that the Tea Party's preferred candidates may be less palatable to the electorate than the moderates they've pushed out. (In what is shaping up to be a great year for the Republicans, O'Donnell looks all but certain to lose her race.)

John Martin, a moderate conservative activist who in 2008 led the "Republicans for Obama" campaign, says unequivocally that the Tea Party will be bad for his party. "A lot of people who are running as independents today were Republicans three years ago," he points out.

Other polls have found that the Tea Party is doing more to fire up horrified Democrats than it is to build Republican support.

Grow, grow, grow your own

There is one more problem with the Tea Partiers: and that is, they don't think much of the Republican Party, either. Many see the party establishment as a bunch of just the kind of elitist career politicians they've set out to destroy. Democratic activists are full of glee [4] that their party looks much more unified in presumed defeat than the Republicans are looking in victory.

This war for the party's soul could do more harm than good – particularly if the wrong side wins. A big Republican victory this year could lead to overconfidence, making it harder for the party to move back to the centre ground where presidential elections are won. And it could make the Republicans more likely to pick someone unelectably right-wing to run against Barack Obama in 2012.

Some activists, you sense, are aware of this problem. Stainbrook is oddly contradictory in his attitude towards the Tea Party.

He is effusive, describing it as "a wonderful, beautiful thing". "I love it," he says. "Why would we not want to nurture and grow something that's ours?" But he cheerfully admits that the Tea Partiers aren't going to stop their attacks on moderate Republicans for the sake of party unity.

And in the same breath with which he heaps praise on the movement, he concedes that it might dent his party's electoral prospects. "There is always the danger," Stainbrook says, "that if in the primary you pick an ultra-conservative then you've got a candidate that's not as palatable to the general public.

"But that's the thing," he adds quickly. "They don't want a moderate any more."

Perhaps the party doesn't. But it might just turn out that the voters do.

Jonn Elledge [5] is a London-based journalist. In autumn 2008 he wrote the New Statesman's US election blog.