The Conservatives' by-election campaign received a boost today with the defection of five Labour councillors.
The group, all from Ealing Southall, said Mr Cameron's attempts to modernise the Conservative party had made it their "natural home".
Mr Cameron announced the defection while campaigning in Ealing Southall with the local Tory candidate Tony Lit.
Menzies Campbell and Jack Straw were also campaigning in the area ahead of the July 19 by-election triggered by the death of Labour MP Piara Khabra.
The Conservative leader announced to the media today that local Labour councillors Manjit Singh, Gurcbaran Singh, Maninder Kaur Keith, Jarnail Singh Jandu and Jagdish Gupta had decided to campaign for the Tory prospective parliamentary candidate.
In a joint statement the five Labour councillors said it was increasingly obvious Mr Lit was the best person to represent Ealing Southall.
They wrote: "At a national level we are extremely impressed by David Cameron's modern Conservatives and, with the changes that he has now made, we feel that the Conservative party now best represents our views.
"In a sense we believe the Conservatives have become our natural home and we are very excited about the opportunity to play a full part in a party which we believe is destined to form the next government."
Mr Cameron spoke of his delight at the move, which came a week after Brij Mohan Gupta, vice-chairman of Ealing Southall Conservatives, defected to the Liberal Democrats.
Mr Gupta said Mr Cameron had replaced the Conservative party's substance with spin, echoing Quentin Davis, who crossed the House of Commons to Gordon Brown's backbenches on the eve of his premiership.
Today Mr Cameron was able to point to the Conservatives' policies, and said British Asians shared the party's belief in family, enterprise and work.
He said: "I think it is a really important moment for Ealing Southall, but also an important moment in British politics.
"This is the clearest possible indication that the Conservative party having been changed and modernised is now a voice for everybody."
In further pre-election movement, two independent candidates have urged their supporters to back mainstream candidates.
Golbash Singh is now urging people to back the Conservatives and vote for Mr Lit.
Kuldeep Singh Grewal, who is a Labour member but stood as an independent after arguing with the local party, also told his supporters to vote for the official Labour candidate Virendra Sharma.
The Liberal Democrats today claimed the Labour and Tory campaigns were descending into chaos and were celebrating being the only party not to lose any supporters.
A by-election is also taking place in Tony Blair's former constituency of Sedgefield.