Automatically expelling foreign criminals would be an attack on equal rights, says a committee that is against both the deportation initiative and its counterproposal.
Certain groups will be subject to special punitive law if either version is accepted, according to Green parliamentarian Marlies Bänziger.
This would be unconstitutional and especially unfair for people who had been born here but had not acquired Swiss citizenship, she told the media in Bern on Thursday.
A lawyer speaking for the group said the legislation would be unnecessary as Switzerland is already strict with foreigners who have been convicted of crimes.
The committee represents 70 leftwing organisations, unions and parties.
On November 28, Swiss voters will have their say at the ballot box.
swissinfo.ch and agencies