Five Labour councillors in Southall, west London, have defected to the Conservative Party.
The by-election follows the death of Ealing Southall's MP last month
The switch by Manjit Singh, Gurcharan Singh, Maninder Kaur Keith, Jarnail Singh Jandu and Jagdish Gupta comes before the Ealing Southall by-election.
Meanwhile, Kuldeep Singh Grewal, an independent candidate for the by-election, has urged his supporters to vote Labour instead on 19 July.
Another independent candidate - Golbash Singh - is now supporting the Tories.
Liberal Democrat leader Sir Menzies Campbell is visiting Ealing Southall with his party's candidate, Nigel Bakhai.
Mr Cameron is doing the same with the Tory contender, Tony Lit.
The by-election was triggered by the death of Labour MP Piara Khabra last month, at the age of 82.
Mr Grewal said he was throwing his "full weight" behind Labour candidate Virendra Sharma.
He said he had always been a Labour member but had decided to stand as an independent following "some internal grievances" with the party.
Another by-election is being held in Sedgefield, County Durham, on the same day, following Tony Blair's decision to stand down as an MP.