Why is immigration the main issue in the Rochester and Strood by-election?

Why has immigration become the main issue on the campaign trail?

Immigration has become the main issue so far in the Rochester and Strood by-election campaign.

It may be surprising to look at the statistics though.

In Rochester and Strood the number of people living there who were born overseas - including Ireland - is 9.3%

That is below the national average of 12.4%. It is also below the South East average, which is 12.1% and below the average of the other Medway towns.

So why has it become such a focus in this constituency?

'Oldest voters'

According to the latest British Social Attitudes survey: "Those most likely to be directly exposed to migration in their daily lives have much more positive views".

It also found the most negative views were found among the oldest voters and those with no migrant friends.

But perhaps it is also because this by-election was triggered by the defection of a Conservative MP to the UK Independence Party and immigration is their main priority.

UKIP want to gain control of the UK's borders and of immigration - they say it is only possible by leaving the European Union.

David Cameron is facing his own European battle at the moment. Many of his backbenchers would like to see the UK leave the EU.

He wants to stay in but is promising to re-negotiate the terms - the recent unexpected bill for £1.7bn suggests that may be easier said than done.

'Concrete changes'

The PM is promising to clamp down on benefits and health tourism, close bogus colleges and make it easier to deport foreign criminals.

Labour know many think they were too soft on immigration while in government and Ed Miliband is now promising "clear, credible and concrete changes" within months if his party wins the general election.

The Liberal Democrats believe immigration is good for the economy but want to bring back control to immigration system with exit checks to track those leaving the country.

And the Green Party want to abandon what they call the "artificial immigration cap" and ensure human rights are respected.

All the parties know immigration has become a significant issue in this by-election - they now have to convince voters their candidate can actually do something about it.