Opposition parties in the assembly are entering a “new-phase” of co-operation in scrutinising Welsh Government, Plaid Cymru AM Adam Price said.
Mr Price said the opposition will soon establish a “common-front” on a range of issues.
The Labour-led government has a working majority in the assembly but recently lost three opposition-day votes.
The leader of the Welsh Tories last month said he wanted to work with other parties.
The UKIP Wales leader Neil Hamilton said that he was prepared to co-operate even if it meant he would “vote through things in which fundamentally I don’t believe, but which are better than the other alternatives”.
Plaid ended its “compact” co-operation agreement with the government last October, just days after the two parties agreed a deal to ensure Labour’s budgets for 2018-19 and 2019-20 would be passed by the assembly.
“I think that we are now entering a new phase,” Mr Price told the BBC’s Sunday Politics Wales programme.
He said: “Where there is legitimate reason to challenge the Welsh Government, not opposition for opposition’s sake, but where we would be failing to do our job if we weren’t to do it, then I think we will see increasingly… a common front on a whole range of issues.”
Opposition parties worked together when the Conservatives and UKIP backed Plaid leader Leanne Wood in a deadlocked Senedd vote to choose a first minister following the 2016 assembly election.
Plaid agreed to back Labour’s Carwyn Jones in a fresh vote the following week.
Mr Hamilton said he had hoped the deadlocked vote “would begin a process whereby collectively the opposition parties… could exercise and overwhelming influence on the Welsh Labour Government”.
“Having bottled out at the last minute, Carwyn Jones was let off the hook by Plaid,” he said.
Despite a so-far muted response from Plaid Cymru and UKIP to his original offer to co-operate made last month, Welsh Tory leader Andrew RT Davies says his door is still open to any party who wants to work with the Conservatives.
“I don’t have any ideological reasons why I cannot work with other parties in this institution, because to me the ideological reason why Wales sadly hasn’t delivered in the first 20 years of devolution is the Labour Party,” Mr Davies said.
Adam Price hopes that Plaid and the Welsh Conservatives could work together over the issue of the proposed M4 relief road.
His theory could be put to the test on Wednesday during a Plaid-sponsored debate on the M4 relief road, calling for an assembly vote on the financing of the project once the public inquiry concludes.
Mr Price said that while his party has a different policy to the Tories, the parties “possibly” both agree that the assembly should have a vote following the end of the inquiry.
He added that the two parties “may find common cause with some rebel members of the government backbench as well”.
Some Labour backbenchers are known to oppose the M4 relief road project.
Labour won 29 seats at the last assembly election – leaving it short of a majority.
Despite adopting two other non-Labour AMs into the government, absences on the government side meant it lost opposition votes in January.
Sunday Politics Wales, BBC One Wales, 11:00 GMT