It follows claims the election is being distorted by "hard left" activists and Conservatives joining the party in order to elect a non-mainstream figure.
Darren Williams, secretary of Welsh Labour Grassroots, said the negative campaigning was "disappointing".
He said members should be allowed to make up their own minds based on the policies of the four candidates.
Former Prime Minister Tony Blair is among those to have warned Labour against moving to the left following a poll that put Mr Corbyn ahead of Andy Burnham, Yvette Cooper and Liz Kendall in the race to succeed Ed Miliband.
Two backbench Labour MPs have called for the contest to be halted amid claims that many of Mr Corbyn's supporters are not genuine, mainstream party members.
Mr Williams, who backs Mr Corbyn, told BBC Radio Wales he objected to such a "high handed" attitude.
"It's disappointing the degree of negative campaigning and mudslinging that's come into this contest," he said.
"[Former deputy prime minister] John Prescott was right to say that this is a decision that ordinary Labour party members and supporters will be making and they should be allowed to make it.
"They should be allowed to listen to the arguments and see the policy differences between the various candidates and make up their minds on that basis."
Torfaen MP Nick Thomas-Symonds, who backs Andy Burnham, told BBC Radio Wales he had not seen any "specific evidence of malpractice" and he would support whoever wins the contest.
"We are all part of the Labour family - we all share the same ideals of social justice," he said.
"Whilst vigorous debate after a defeat like this is very important, at the end of this contest we have to weld back together and have that central aim of getting a Labour government in 2020, whoever our members and registered supporters choose as leader."
"Mudslinging" in the Labour leadership contest should stop, a Welsh supporter of left-winger Jeremy Corbyn has said.