Whose side are you on? It is one of the most powerful questions in politics and Ed Miliband believes it is the key to seeing him installed in Downing Street.
That's why Labour is targeting the so-called non-doms today. They're very rich, often foreign and enjoy a lifestyle that makes them resented by anyone who's struggled to make ends meet in recent years.
That is, incidentally, the same reason George Osborne targeted them to pay more when he was in opposition and increased the annual tax charge some pay in his last Budget.
Let's be clear though, these people are not tax dodgers. They pay tax on their UK earnings plus an annual charge of £30,000 or more to have a totally legal tax status that Gordon Brown as well as George Osborne decided to keep as they were advised they risked losing more in tax by scrapping it than keeping it.
None of that matters in the election though. The non-doms have been selected by Ed Miliband and Ed Balls as a symbol of what is wrong with Britain today and a symbol of their intention to break with their own past.
Some will say that this won't trump the good economic news which David Cameron sees as the key to his survival at Number 10.
They are forgetting the lessons of another election.
Barack Obama won a second term at the White House despite being very unpopular and despite being seen as less economically competent than Mitt Romney. Why? Because he deployed that question against an opponent he successfully portrayed as the out of touch friend of the super-rich.
Update, 16:53 BST
Will Labour's proposal to scrap the special tax status for so-called "non-doms" cost the country money - as Ed Balls appeared to be saying just a few weeks ago - or will it raise money, as he and Ed Miliband are saying today?
The truth is that no-one knows and no-one can know because all the figures being bandied around today are based on guessing how rich foreigners might behave in the future. It's a guess not just about whether they'll stay here or leave but what they and their tax advisers will do with their money to stop the taxman getting the hands on it.
The Balls confusion seems to stem from the fact that Labour wanted and got headlines today which said they would scrap non-dom status altogether (which Ed Balls said would lose the country cash) when, in fact, their policy doesn't end the idea of a special tax status (even if it's not called a "non-dom" rule) for those who live here temporarily.
This announcement was always more about the headlines than the details; the politics than the economics; the symbolic rather than the substantial.
It was meant to pose what I earlier called the most powerful question in British politics: Whose side are you on?
The surest sign of that is the fact that the Tories have not argued that taxing non-doms more is a bad idea. Indeed, they've claimed they're doing it already. They are just mightily relieved to be able to talk instead about chaos and confusion and sums not adding up.