The Federal Council revealed on Wednesday its proposal to buy 22 Swedish-made Gripen planes to replace its ageing F5 fighters.
Socialists fear the deal will mean a rise in military spending that could translate into budget restrictions in other departments, notably education, transport and agriculture.
Meeting in Lucerne on Saturday, party members unanimously passed a resolution on the matter after a document outlining the group's position was approved in October, ATS news agency reported.
The party rejects the prospect of a spending rise all the more because there was no public vote on the plane deal, described as a "scandal" by National Council (parliament) member Eric Voruz.
It will launch a referendum if parliament seeks a legal base to justify budgetary restrictions or requests a rise in military spending to cover the acquisition.
If such a referendum does not halt the deal the socialists will call for a moratorium on the purchase until 2025.
The Gripen deal will be put to parliament as part of the government's 2012 arms programme.
Parliamentary decisions can be put to a public vote in Switzerland if a sufficient number of votes are collected.