Tony Abbott has been left red-faced after a member of his opposition frontbench sought the ''tainted vote'' of suspended Labor MP Craig Thomson.

Liberal senator Mathias Cormann has admitted he sent a letter to Mr Thomson asking the independent MP to support a Coalition bill to amend government changes to superannuation.

Vote not wanted ... Craig Thomson. Credit:Andrew Meares

The letter was sent to all crossbenchers and Senator Cormann asked the MPs to ''seriously consider'' the issues raised by the Coalition, saying they were ''worthy of your support''.

Senator Cormann said today it was a clerical mistake that should never have happened.