The Opposition leader is travelling to the Hampshire town where the Lib Dems are engaged in a fierce battle to prevent their Tory coalition partners snatching the seat.
Labour is hoping for a good showing in the contest sparked by the resignation in disgrace of ex-cabinet minister Chris Huhne after he admitted dodging a speeding penalty.
But despite recruiting best-selling author and satirist John O'Farrell as its candidate, insiders acknowledge it has no realistic chance of winning on February 28.
Mr Miliband will use his visit in a bid to capitalise on his surprise announcement yesterday that Labour favoured reviving the 10p income tax rate.
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Charities warn that sitting down too long can lead to health risks such as type 2 diabetes and cardiovascular disease.
The Labour leader will use a speech to set out the differences between his party and the Conservatives with particular emphasis on the NHS.
Kate will join Prince William on a tour of the Stephen Lawrence Centre before visiting a charity working with disadvantaged young people.