Plaid Cymru should "come together" with UKIP to "take Wales forward", according to Neil Hamilton.
UKIP's assembly group leader accused Plaid of not being a real opposition party but "Labour's lapdog".
In the 2016 assembly elections, Labour won 29 seats, Plaid 12, the Conservatives 11, UKIP seven and the Liberal Democrats one.
Plaid Cymru leader Leanne Wood dismissed the statement from Mr Hamilton as a "dead cat deflection".
Mr Hamilton called for Plaid to appoint a leader willing to work with UKIP.
The AM for Mid and West Wales said he was responding to suggestions by Plaid's Rhun ap Iorwerth that it would go into coalition with Labour in the future if circumstances allowed.
Mr ap Iorwerth said on Tuesday he would consider standing as Plaid leader, if Leanne Wood were to step down.
Mr Hamilton blasted Mr ap Iorwerth and Ms Wood, saying: "Plaid will never progress under these faint hearts.
"They need a red-blooded leader like Adam Price or Neil McEvoy, both of whom are very effective AMs, hungry to break the cosy Cardiff Bay consensus."
Mr Hamilton added that "as a real nationalist party", UKIP would be "pleased to work with Plaid Cymru to take Wales forward".
He said Mr McEvoy recognised Labour had "failed Wales' working class".
"Real opposition means more than timid tub-thumping," he added.
"Opposition parties should come together to smash the tired and corrupt consensus which has turned Wales into the poorest part of the UK."
In response, Ms Wood tweeted: "Plaid Cymru is the home for all who are fed up with Labour misrule and want change".
She described Mr Hamilton's comments as a "dead cat deflection" from his own party's ongoing leadership issues.
Eleven candidates are contesting UKIP's third leadership contest in a year, with the party's MEP for Wales Nathan Gill among those distancing themselves from anti-Islam candidate Anne Marie Waters.