Devon County Council's leader has urged Cornwall Council to work with him on getting more power for the region.
John Hart said Cornwall, Devon and Somerset would have a stronger voice together.
It comes after the government said Cornwall should be allowed to make the case for greater powers on issues such as transport, planning and health.
Adam Paynter, deputy leader of Cornwall Council, said he was "very excited" about the government's offer.
Cornwall Council has published its list of demands for more powers.
But Devon has come up with a plan to work with Somerset and wants Cornwall to join in.
Conservative Mr Hart said: "Devolution can give us the power to take our decisions and not have to be reliant on what London tells us.
"But Cornwall is not an island and if it works with us I am convinced we can talk much more seriously to London about devolving powers to the South West."
Liberal Democrat Mr Paynter said Cornwall's demands risked being "diluted" by working with Devon and Somerset.