The ex-adviser to Tony Blair urged pro-EU supporters to vote tactically to get another Brexit referendum.
Speaking on the ITV News election podcast, Calling Peston, Mr Campbell said: “I will go to [South West Hertfordshire] and campaign for David Gauke, I would go and help Philip Lee, I think we’ve all got to do what we can do…
"It really depresses me, why are the Greens standing against Anna Soubry.”
Mr Campbell told the podcast that he would be out actively campaigning for Mr Gauke and Dr Lee.
Former Conservative minister David Gauke and Tory-turned-Lib Dem candidate Dr Philip Lee are both standing on anti-Brexit platforms.
Mr Campbell was expelled from the Labour Party in May after voting for the Liberal Democrats in that month's European elections.
In September, Dr Lee crossed the floor of the House of Commons to sit on the opposition benches, wiping out Boris Johnson's working majority, ahead of a vote which resulted in the blocking of a no-deal Brexit.
It was the same vote in which Mr Gauke rebelled against the government, which saw him and 20 other Conservatives expelled from the party.