After its triumph in the European elections in May, Ukip was stuck in the doldrums before the backbencher Douglas Carswell electrified politics by resigning his Tory-held seat in Clacton and announcing his intention to stand for the United Kingdom Independence Party in an unexpected by-election.

Meanwhile, in the USA, the Tea Party is up one week, down the next. But the difference isn’t really between how well the insurgent parties are doing. It’s between the British and American political systems that are shaping their fate.

Though they’re compared relentlessly, Ukip and the Tea Party are actually very different animals. Ukip’s backers are more likely than the average voter to be older, male, and lower middle class former Tories.

While Tea Party supporters tilt conservative, pollster Gallup has found that, demographically, they’re average Americans. In other words, Ukip has something of a class identity, whereas the Tea Party is essentially an ideological movement. And while both oppose the political establishment, Ukip attacks the Westminster all-party consensus, while the Tea Party offers a clearly conservative alternative.

Because it’s ideologically so well-defined, it’s easy – especially for the many journalists who don’t like conservatives – to fall into the trap of talking about the Tea Party as though it is, in fact, a party. But the last new political party formed in the US was the Republicans, in the mid-1850s.

Remarkably, the US party political system is far older and more stable than its British counterpart. Since the 1850s, Britain has gained and lost the old Liberal Party and added Labour, the Liberal Democrats, and now Ukip. In the UK, if you want to break into the political system, you form a new party.

In the US, that doesn’t work. There, rising ideologies are incorporated into the existing parties, which shift their positions over time. In the 1890s, progressivism remade both major parties. Not until the administration of Franklin Delano Roosevelt did it clearly settle on the Democratic Party as its home.

Much the same was true of neo-conservatism in the 1970s, which started as a rebellion on the left and ended up as an influential part of Reagan’s coalition. The Tea Party was never going to be a truly new party. Like past movements, it was going to remake the system by becoming part of it.

But you don’t remake the system by asking nicely. You do it by getting involved, and sometimes that means getting dirty.

It’s easy to play the horse-race game. When a opponent in the Republican primary knocked off House Majority Leader Eric Cantor in Virginia, the Tea Party was up; when Republican senator Thad Cochran in Mississippi survived a similar challenge by the narrowest of margins, it was down.

Of course, those wins and losses matter. But the full impact of the Tea Party won’t be felt for years. Recall Barry Goldwater’s quixotic campaign in 1964, which met a devastating defeat at the hands of Lyndon Johnson. But in losing, Goldwater laid the foundations for Ronald Reagan’s triumph in 1980.

The Tea Party will exercise its influence by bringing new candidates into new races, and by pushing old ones to the right. It won’t take over the Republicans; it will remake the Republicans.

The reason it works this way in the US is because the American system isn’t a Parliamentary one. British parties need candidates who are loyal, because a Parliamentary party – especially a governing party – can’t survive with many disloyal MPs. John Major learned this in the 1990s.

In the States, parties are weaker, so candidates need to mobilise voters directly. And that means both voters and candidates gain from the primary process. Voters get more choice, while the candidate that survives the intra-party fight brings an energised base to the general election. Without primaries, the US would look a lot more like Britain, and the Tea Party would be, like Ukip, a new and separate party.

The slide in Ukip’s ratings – down almost to single digits – is striking, but not remarkable. Ukip insiders knew their party would tail off after the European campaign.

The irony is that the events of the past month have been good for Ukip. David Cameron’s reshuffle showed yet again that he lacks a firm commitment to any obviously conservative principles. A stronger Prime Minister would have stood by Michael Gove, not demoted him.

Even more damagingly, Cameron’s failure to stop Eurofederalist Jean-Claude Juncker from becoming President of the EU Commission has made nonsense of his promise of an EU referendum. And now the Carswell defection.

But unlike the Tea Party, Ukip cannot comfort itself with analogies to Barry Goldwater. If it doesn’t poll equal with the Lib Dems in 2015, at a minimum, it will have failed.

 - Ted R. Bromund is a senior research fellow in Anglo-American Relations, based at The Margaret Thatcher Center for Freedom at The Heritage Foundation in Washington.