David Whipp was caught on domestic CCTV removing the flier in Barnoldswick.
The film of the Lib Dem group leader on Lancashire County Council and Pendle Council has been posted on social media.
Cllr Whipp, who is standing for re-election to both authorities, defended his actions as politicians from other parties reacted to the footage.
County Cllr David Whipp, Liberal Democrat group leader on Lancashire County Council, was seen depositing a Lib Dem leaflet before removing a Conservative one - he has since returned it
He said: I haven t stolen anything. A flyer was stuffed in the top of a post box blocking the opening.