At one level the diplomatic protocols to be observed by a Prime Minister towards foreign elections are pretty straightforward. Stay out of it is rule Number 1. Since you can't predict who will win and will have to do business with the victor regardless of preference there is no benefit to be had in naming a favourite.
Easier said than done. John Major famously did himself no favours by conspicuously fancying George HW Bush over Bill Clinton. More recently, David Cameron made relations with new French President Francois Hollande needlessly tricky by advertising his hope that Nicolas Sarkozy would hold onto the job. (An error that Ed Miliband has this week exploited to fairly good effect.)
Mindful of the need not to repeat the mistake, Cameron will tomorrow host Mitt Romney, the Republican candidate in this year's US presidential election, in Downing Street. Number 10 has some repair work to do after learning that Cameron's effusive praise for Barack Obama earlier this year was judged unseemly and excessively partisan in the Romney camp. (Ed Miliband will also meet Romney but no-one expects that to be anything other than a token making of acquiantence.)
There is a residual notion around on both sides of the Atlantic that Republicans have a natural affinity with Tories and Labour partner up with Democrats, although the evidence doesn't really support that view. Not recently in any case. There is, of course, the famous intimacy between Tony Blair and George W Bush as a glaring counter-example. Meanwhile the Cameroons' enthusiasm for Obama is unfeigned - approaching something like star worship, although that has as much to do with admiration for the incumbent President's brilliant campaigning style as his political inclinations.
Senior Tories are wisely staying tight-lipped about their hopes for November's poll. There is one obvious reason why they might be glad to see Romney prevail. Obama's economic strategy is, crudely speaking, closer to the stimulus-driven Keynesian prescription for responding to economic malaise than Cameron's reliance on instant, harsh fiscal retrenchment. Labour likes to hold up the growing US economy as proof of the fact that raw austerity is the wrong plan. By extension it should stand to reason that, if Obama is ejected and his economic plans deemed to have failed, Cameron can feel mildly politically vindicated. Ultimately he will want conservatism to be victorious in as many jurisdictions as possible.
But that view, I think, underestimates how far removed the US Republican party has become from what passes as normal political discourse in this country. Romney may be the most moderate candidate the Republicans can muster but the is no disguising the fact that the party's centre of gravity has shifted in recent years to terrain that qualifies as way off to the right of where David Cameron would like the Tories to stand. The "Tea Party" tendency, with its obsessive dogmatic hostility to Big Government, its fixation on the pursuit of anti-liberal culture wars and its nurturing of Christian religious fanaticism has pretty much nothing to offer a British political movement wanting to be taken seriously.
In their book It's Even Worse than it Looks US political commentators Norm Ornstein and Thomas Mann memorably describe the Republican party as "an insurgent outlier – ideologically extreme; scornful of compromise; unpersuaded by conventional understanding of facts, evidence, and science."
Even if a President Romney were to distance himself from the Tea Party, a Republican-led US would surely become ever more culturally and politically alien to Britain. There would be no advantage - and some hazard - for Cameron in being perceived as leading the cousin conservative party on this side of the Atlantic.
Meanwhile, a separate problem for Cameron is the perception that the ongoing global economic crisis is deadly to incumbents. Sarkozy's demise was generally seen as a straightforward decision by the French electorate to sack the person in charge of a failing economy. If Obama loses it would be for pretty much the same reason. It is quite possible that, historical party alignments aside, Cameron would feel more comfortable seeing his old barbecue buddy Barack survive than see another fellow leader felled by the crisis and replaced by a man who stands for a brand of conservatism than many in this country think of as plain nuts.