Labour has accused the Conservatives of "descending into the gutter" after the party raised issues of Ed Miliband's character and his relationship with his brother David as part of the debate over nuclear weapons.
Michael Fallon's comment that "Ed Miliband stabbed his own brother in the back to become Labour leader. Now he is willing to stab the United Kingdom in the back to become prime minister" prompted the Labour leader to accuse the Defence Secretary of demeaning his office.
The row is the latest incident in a campaign in which Tories have cast the General Election as a straight choice between the Labour leader and David Cameron as prime minister.
Mr Cameron has branded Mr Miliband "despicable" and "useless" and taken several opportunities to remind voters of the fratricidal struggle which won him his job.
Mr Cameron launched a direct attack on Mr Miliband from a lectern outside 10 Downing Street immediately after returning from Buckingham Palace to inform the Queen of the dissolution of parliament. He later defended his approach, saying: "Some people might say, don't make this personal. But when it comes to who's prime minister, the personal is national."
Tory strategists regard the personality issue as a key asset in the election battle, as Mr Cameron regularly outpolls his Labour rival by a wide margin as best candidate for PM, even when Conservatives are lagging behind as preferred party. They are aware that many voters know little about Mr Miliband beyond the fact that he sometimes appears awkward in photos and that he fought his own brother for the Labour leadership.
Labour hopes the election campaign will allow their leader to shake off these images, while Conservatives are determined not to let him do so.
Elections expert Professor Matthew Flinders warned that highlighting Mr Miliband's supposed ruthlessness in challenging his brother could backfire on the Tories.
"The problem with this, however, is that research suggests that the public want a strong, decisive leader so the Tories need to be careful not to shoot themselves in the foot," he said.