A Liberal Democrat AM says a new coalition with Labour in the Welsh assembly would bring "irreparable harm" to his party.
Rhodri Morgan and Mike German were in coalition from 2000-2003
Peter Black said there should be no talks with a "rejected" Labour party, and his views were shared by a "very strong body of opinion" among Lib Dems.
He also called for a new assembly group leader to replace Mike German.
But he was accused of being "irresponsible" by fellow Lib Dem AM Eleanor Burnham.
Lib Dems, who won six assembly seats for the third consecutive election last Thursday, are viewed as the most likely coalition partners with Labour's 26 AMs.
However, the four Lib Dem leaders of councils in Wales have already said they are opposed to teaming up with Labour.
Mr Black, a South Wales West AM, told BBC Wales' The Politics Show that he represented a very strong body of opinion in the party against a coalition and he did not think a deal with any party would help.
He also said Mr German had been an outstanding leader, "but it comes to the stage where we have to move on," and it was time for "a fresh approach".
But Lib Dem Cardiff Council leader Rodney Berman said it was not helpful to be talking about ousting Mr German at this stage.
Mr Black has also written on his weblog: "We obviously want to put Liberal Democrat policies into effect but there are other ways of doing that apart from being in government.
"The last thing we need is to enter a divisive and difficult process of negotiation with a Labour Party that has been rejected by the people of Wales when our own problems remain unresolved."
Later, he wrote that while Mr German had a right to stand in an election, "we need a leader with that fresh mandate if we are to change."
He went on: "I also believe that entering a coalition now would do irreparable harm to the party. It would prevent us from developing a fresh image and refining our policies and principles."
But North Wales AM Ms Burnham said: "The rest of us in the assembly group disagree with him," she said.
"We believe we should be concentrating on sorting out any deal with any other party and to do the best by the commitments we made in the manifesto," she said.
"Anything else should be a secondary issue."
Ms Burnham said Mr Black should not be raising the leadership issue.
"I think Peter Black is being irresponsible," she said. "He's in a minority, I assure you.
"We are trying to be measured and responsible. He should consider his own position."
Plaid Cymru is the second biggest party with 15 seats and leader Ieuan Wyn Jones has not ruled out an agreement with Labour.
Plaid AM Dafydd Elis Thomas said it might be possible for Labour to govern with 26 seats.
He said that "it might not be comfortable but it might be good for the institution."
Lord Elis Thomas, who has been the assembly's presiding officer since it first sat in 1999, said he was not actively involved in coalition talks because he was still in that post.
But he said: "Specific measures and policy decisions could be supported by other parties. And I'm sure that we as Plaid would like to be in that position so that we could influence government."
"Privately it would be my favoured option and I think that would put us in the stronger position as a party for future development."
He told BBC Wales' Maniffesto programme what sort of price he believed Labour would have to pay for any such arrangement.
He said "the constitutional development" was the important issue, and that the assembly should have full law-making powers in the next three to four years.
But he said others in Plaid may have different views.