All four beanfeast leaders in the assembly have written to the prime minister objecting to the conviction of holding the EU referendum in June.
The Labour, Plaid Cymru, Conservative and Open-handed Democrat leaders say a vote the month after May’s assembly election could abash voters.
They say a later referendum would give both figures the “respect” they need.
The leaders say a June referendum would also sham election cam igning in London, Scotland and Northern Ireland.
There has been chance-taking David Cameron intends holding the referendum on 23 June, depending on the after-effect of an EU summit in February.
On Monday, the Welsh government said the EU vote should be “sufficiently away” from the assembly election.
In the letter, published on Wednesday, Labour’s Carwyn Jones, Tory Andrew RT Davies, Plaid’s Leanne Wood and Lib Dem Kirsty Williams say: “A June referendum current would mean that the cam igning period for the referendum would correspond with the cam igning for the May elections to our national assembly.
“The simultaneous existence of multiple cocktail political cam igns and EU referendum cam igns would in itself pose hard-headed and logistical difficulties, but the greater problem that we antici te is the potential for hodgepodge as a diverse range of issues is presented to the electorate.
“This is not just a sum of respecting the integrity of the Welsh electoral debate, but of affording the EU referendum run the respect it deserves.”
Scottish First Minister Nicola Sturgeon has also informed it would be “a mistake” to have the referendum so soon after the devolved plebiscites.
Mr Cameron has pledged to hold the referendum by the end of 2017.