Plaid Cymru leader Ieuan Wyn Jones has blamed the party's exclusion from the televised leaders' debates for not gaining more seats.
Plaid had hopes of gaining Ynys Mon and Llanelli but Labour won both, while Liberal Democrats also held Ceredigion.
However, by siding with the Scottish National Party (SNP), Plaid are now in a position to offer support on a vote-by-vote basis to a hung parliament.
Mr Jones said their exclusion from the debates affected their results.
"There is no doubt that our hopes of gaining additional seats were dashed during the campaign as a result of the portrayal of this election as a three party contest in Wales by the UK media.
Upbeat and optimistic
"Although we welcomed the televised debates we knew from the outset that the decision to exclude Plaid and the SNP from that important UK-wide media platform would put us at a disadvantage and that has proven to be the case."
"Despite this we remained focused, upbeat and optimistic".
The party's Hywel Williams did win Arfon.
However, Lib Dem Mark Williams won in Ceredigion with a 8,324 majority a 10.5% swing from Plaid.
Plaid had also hoped to win Ynys Mon - the seat held by Mr Jones in the Welsh assembly - but it was retained by Labour's Albert Owen.
Plaid also suffered a blow in Llanelli, which was held by Nia Griffith for Labour.
Plaid Cymru have taken the newly created constituency of Arfon
However, Plaid retained Carmarthen East and Dinefwr, where the candidate was Jonathan Edwards after sitting MP Adam Price stood down.
"I think the sense is that we have been squeezed," Mr Jones told BBC Wales.
"There's no doubt about it. I think the context of the election has clearly been set early on with a concentration on the leaders' debates.
"We have been quite badly squeezed in some areas."
He said he believed the same process had happened to the SNP and added: "It's not the sort of breakthrough we would have wished for. It's probably going to be a very disappointing night for us."
"Where we have campaigned very hard it looks as though we may make some inroads, but at the end of the day it looks as though we have been squeezed and I believe a similar pattern has happened in Scotland.
"I think that the election has been quite clearly dominated by the leaders' debate, and the first debate set that context and set the real dynamic, and I think that is going to have a real impact."
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