The prime minister issued a brutal attack on the BBC today, amid accusations that Tory candidate Maria Hutchings ducked out of a radio debate with other candidates.
The Eastleigh candidate said she missed the BBC Radio 5 Live debate because she had to attend a factory event elsewhere in the constituency, but critics said it was an effort by the party machinery to keep her out the spotlight after a series of gaffes over gay marriage, Europe and private schools.
"I think the BBC has behaved badly and stupidly about this from everything I've heard," Cameron told a BBC journalist.
"My understanding is that we were discussing with you for ages about the timing of the hustings and the fact that Maria wanted to be with me for this meeting.
"Don't get involved in a great argument about form, look at the substance.
"I do think this is a totally got-up thing by the BBC. You're not the most important thing in this by-election. The candidates are."
The Liberal Democrats seized on Hutchings' absence as proof the party had lost faith in her.
"The Tory candidate is clearly running scared," Liberal Democrat president Tim Farron said.
"She’s refusing to show up to a hustings with the people she says she wants to represent, proving her claims to stay in touch are empty and worthless.
"You have to ask why the Conservative machine keeps trying to hide her away. This is a candidate who says on her leaflets that she puts local people before political ambition, yet the moment she gets to share the spotlight with her party leader all that is forgotten."
Members of the audience expressed anger at Hutchings failure to appear during the radio debate.
"I'm very disappointed. I've got lots of questions about small businesses in Eastleigh that are closing down, we want to know the policies," one audience member said.
Another said: "I am very disappointed and angry she won't be able to answer specific questions about Hamble [a local town]."
A third member of the audience said: "I'm disappointed Maria not here because I wanted to quiz her on how good local secondary schools are."
Hutchings has proven a colourful and outspoken character since the campaign began, with decidedly right-wing views on gay marriage, Europe and abortion.
The selection of the candidate who failed to win the seat off Chris Huhne in 2010 baffled many political analysts.
Some have interpreted her selection as a sign the Conservatives' national leadership has lost control over the party, in a manner similar to Labour in the 1970s and 1980s.
Analysts presumed the decision to keep Hutchings away from the debate was an effort to stop her making more controversial statements in a live radio format.
If the Tories fail to take Eastleigh next week, it is likely to encourage rebellious backbenchers to make life difficult for David Cameron and George Osborne.
Party strategists will interpret the loss as a sign the Tories will fail to take 20-odd target seats off the Lib Dems in a general election, making it almost inconceivable the party can win a parliamentary majority.
London mayor Boris Johnson appeared to show signs of that frustration yesterday when he toured the constituency, reportedly muttering "sodding Lib Dem voters" while leafleting and then shouting "have some bumpf" when passing campaign material to voters.
Lib Dems have been making solid use of their strong local control of the area, with detailed information about where their supporters are and a disciplined approach to canvassing.
The party's operation is built on a get-out-the-vote strategy rather than an appeal to new voters.
A win in Eastleigh would give the party a much-needed shot in the arm, proving it can defend its bastions against challengers and that 2015 may not signal its extinction.
It would prove that local campaigns can overcome the gloomy national perception of the party and make it much less likely that Nick Clegg will face a leadership challenge from another party figure, like Farron or Vince Cable, before the next election.
But the party was still facing anger over Chris Huhne this morning, as several members of the audience asked why the party would not apologise for the former energy secretary, who pleaded guilty to perverting the course of justice.
"Let me put this to rest right now," Lib Dem candidate Mike Thornton said.
"Chris Huhne should apologise to everyone in this room and he should do it now.
"If anyone thinks I think any different they are completely and utterly wrong. I taught my daughter to apologise when she's wrong and I think that's what he should do."
Meanwhile, Labour is struggling to benefit from its use of John O'Farrell, the satirist it hoped would boost its performance in the area.
Instead, Ukip is expected to take third place, capitalising on the constituency's predominantly working class voters in the town centre and taking middle class voters from the Liberal Democrats and Conservatives.