He told BBC Radio 4's Today programme his party needed to run "a better election than we've ever had before" in order to make a breakthrough.
Plaid Cymru leader Leanne Wood told the programme she agreed the party need to run a "better election campaign than we ever have before" to secure more MPs.
Plaid  launches its election manifesto in Bangor on Tuesday.
The peer, who led Plaid Cymru from 1984 to 1989, said the party had to achieve what the SNP had in Scotland in showing they were an alternative to Labour.
"I have no issue with the decision of the Welsh people to vote for the Labour Party, because they clearly haven't been convinced that we are a better alternative." he said.
"In Scotland the SNP have convinced them, it seems to me from the polls, and therefore that's our responsibility, we have to have a better election than we've ever had before."
Lord Elis-Thomas admitted Plaid Cymru's election message was not yet clear because of uncertainty over the election result, but he said the "necessary outcome" was "full powers for the National Assembly for Wales and transformation of the relationship with the UK Treasury".
Responding to the comments, Ms Wood told the programme: "In a way Dafydd Elis-Thomas is right, if we are to secure more MPs then we ever have before then we have to run a better election campaign than we ever have before."
She also re-iterated her party's opposition to the replacement of Trident saying: "There is no way that Plaid Cymru MPs will ever vote to support the replacement of a system which would cost Â£100bn and which no government would ever realistically use."
Plaid Cymru has not yet convinced Welsh voters to switch from Labour, former Plaid leader Lord Elis-Thomas has said.