The Tories are hoping to take the Hampshire seat from their Liberal Democrat coalition partners in a contest triggered by the resignation of disgraced ex-Cabinet Minister Chris Huhne, following his conviction for evading speeding points.
In a question and answer session at B&Q’s UK headquarters in Eastleigh, Mr Cameron described Tory candidate Maria Hutchings as a “decent person who tells it like it is”.
Ms Hutchings has, however, already ruffled feathers in the Conservative high command by speaking out against Mr Cameron’s plans for gay marriage and suggesting she would vote for Britain to leave the EU.
Nick Clegg has already made his first foray into the Hampshire constituency, with further campaign visits expected – underlining how high the stakes are for both leaders.
A win for the Conservatives would help to convince restive Tory backbenchers that Mr Cameron still has a chance of leading them to outright victory at the next general election.
Equally Mr Clegg needs the Lib Dems to hold the seat if he is to reassure supporters that they are not facing a catastrophic collapse the next time the country goes to the polls.
The Prime Minister insisted the result of the by-election would not change the Government, but said it was important for the people of Eastleigh to elect the best candidate to represent them in the Commons.
Mr Cameron said: “The most important thing is who knows this area, is going to stand up for it and is a thorough, decent person who tells it like it is? That is what you’ll get with Maria.”
Among the other candidates are John O’Farrell (Labour) Mike Thornton (Lib Dem) and Diane James (UKIP).