The Cameron Tories are a bellwether for Conservative movements in a number of countries, including the US. If they succeed, they will prove a powerful model for many moderate Republicans who believe their party is in an earlier post-Major phase - angry, divided and negative.
If David Cameron fails to defeat Gordon Brown, few Republicans will look across to their British cousins for inspiration. The party will eschew any modernising project for a while longer and stick to their equivalent of IDS.
In this scenario, the Republicans will pick up some congressional seats and governorships. But they will lose the next race for the White House and struggle to take a majority in the House of Representatives.
For elections, even in America, are won in the middle - even if the US middle is well to right of any European middle. Such politics makes activists fulminate - Coffee House is testament to that - but they are "the only thing that works", to quote what a senior Shadow Cabinet member told me recently.
Sure, activists have to be placated - you need them to vote and work to get others to vote. But it is far more important to grab middle-of-the-road voters - even if you come at them from the right.
To work though, such right-hailing, middle-grabbing politics need to be formulated by sensible-looking, human-sounding people. People who don't look like they would foam at the mouth. People who disagree with their opponents but who do not hate them or wish them ill. People like David Cameron.
If, however, the Tory leader fails on 6 May, it may take the Republican Party a while longer to accept this basic fact of 21st century politics. To the detriment of the US. So Dave, go out there and win one for the Gipper.