Immigration - what is the impact on British workers?
"British jobs for British workers" would be one of the most popular slogans any politician could deploy.
If, that is, Gordon Brown hadn't already used it, been accused of using BNP rhetoric and attacked for promising something he simply couldn't deliver.
Politicians like Theresa May are more careful in the language they use and have found other ways to show they empathise with public concern about "immigrants taking our jobs".
The home secretary has highlighted research which showed that that public anxiety might be true. When the government's official migration advisers came up with a startling finding in 2012, she seized on it.
"We asked the Migration Advisory Committee to look at the effects of immigration on jobs, and their conclusions were stark," she said.
"They found a clear association between non-European immigration and employment in the UK.
"Between 1995 and 2010, the committee found an associated displacement of 160,000 British workers. For every additional 100 immigrants, they estimated that 23 British workers would not be employed."
At the time, economists such as Jonathan Portes of the National Institute of Economic and Social Research warned that this figure was out of line with other research; was just one of many statistics in a complex study; could be dramatically altered by moving the date range measured by a single year and should, therefore, be treated with caution.
It was a view shared by many economists within government.
They have now carried out what's been described to me as a "comprehensive literature review" - in other words a study of all the studies into the so-called "displacement effect" of immigration.
It concluded, surprise surprise, that the 23 figure quoted by Theresa May was, indeed, an outlier and, I am told, suggests the true figure is "virtually nil".
Newsnight's Christopher Cook revealed last night that this study of studies has not been published despite being ready to go.
Those who like its finding say it's being suppressed. Those who don't, say the study is not quite finished.
No doubt, the argument going on in Whitehall is about what conclusions should be written at the end of some pretty dry economic tables and statistics and the timing of its release.
What this tells you is two important things:
1. Measuring the economic effects of immigration is difficult but most economists continue to insist that overall immigration increases the size of the British economy and the number of jobs available for British citizens
2. The rival camps in government - the anti-immigration Tory-led Home Office and the pro-immigration Lib Dem-led Business department - will seize on any stat and brief against each other in an effort to prove their case.