Plaid Cymru has said there are "serious problems" with the EU, but Britain must remain a member to help solve them.
Launching Plaid's "Wales In Europe" campaign, Euro-MP Jill Evans said Wales benefits "economically, socially and culturally" from membership.
Party leader Leanne Wood called for a positive campaign and said she was happy to work with other parties.
UKIP said such sentiments were "ironic", when Plaid wanted an independent, self-governing Wales.
David Cameron is hoping to secure agreement on new terms for Britain's membership of the EU to be voted on in a referendum, possibly in June.
Ms Wood had previously accused First Minister Carwyn Jones of failing to present a "positive vision" in his head-to-head debate with UKIP leader Nigel Farage.
In a speech in Rhondda, she warned there would be no "idyllic isolation" if the UK left the EU, urging people to vote for Britain to be "a full participant" in a "new Europe".
Arguments over statistics are unlikely to be enough to convince voters to stay in the EU, she argued.
"It is not possible in 2016 to simply pull up the drawbridge and retreat to an idyllic isolation.
"And even if it were, surely that isn't the kind of country we want Wales to be.
Ms Wood said she wanted a "decentralised, partnership of equals" as "the basis for the new Europe".
Jill Evans added: "We want a stronger voice in Europe but we can't lose the voice we have now.
"There is a lot we'd like to change but we can only do that from within."
Sam Gould from UKIP Wales responded: "What's most ironic here is that Plaid's primary purpose is to campaign for an independent and self-governing Wales.
"When we leave the EU we'll be able to have control, and many of the areas such as fishing, farming, regional regeneration and trade will be be devolved here to the Senedd.
"Is it that Plaid Cymru just want to see the Senedd become a rubber stamping centre for Brussels?"