Violent offenders who pose a threat to society could face being locked up for life with no chance of release.
Parliament has recommended that the issue should not be put to a nationwide vote, but supporters say they will ensure it reaches the ballot box.
On Friday, the Senate followed an earlier decision by the House of Representatives in overwhelmingly voting against the proposals of a people's initiative.
Supporters of the initiative want offenders found guilty of sexual and violent crimes to be handed lifelong sentences based on a psychological assessment at the time of conviction.
They argue that this one-time-only assessment is the only way of safeguarding society.
"[The protection of society] is more important than trying to reintegrate those people back into society," Swiss People's Party parliamentarian Ulrich Schlüer told swissinfo.
"There are some crimes where... it's more important [to make sure] an offender can never again commit a similar act, rather than run the risk of trying to rehabilitate someone when we can't be sure he's really changed," he added.
The government has already drafted legislation for revision of the criminal law, which it says ensures that society is protected from all psychologically disturbed offenders.
It relies on the principle of high-security institutions and therapy, with the chance of an offender eventually being rehabilitated back into society.
The existing system, says a professor of criminal law, Marcel Niggli, already safeguards society, because offenders who are found to be unfit for release remain imprisoned.
Niggli argues that the new proposals could result in a miscarriage of justice because they limit the number of psychological assessments to just one.
"Errors are made in absolutely every human sector and it is absolutely possible that an assessment doesn't yield results that correspond to the facts," he told swissinfo.
"Of course, it is possible that offenders will or will not respond to therapy, but the problem with the proposals is that they don't take into account that a mistake could be made in an initial assessment," he added.
The Swiss justice minister, Ruth Metzler, also insists that convicted offenders must be given the chance to prove whether they can be rehabilitated - a move the initiative rules out altogether.
The campaign for tougher legislation was launched by a group of victims of violence or violent sexual abuse, and their families.
They collected over 194,000 signatures to submit the people's initiative - almost double the number necessary.
Even with both governmental and parliamentary opposition, Schlüer is convinced that the proposals will go to a nationwide vote.
"I know the group of women who have undertaken the whole campaign," he said. "They are people who've had personal involvement in violent and sexual cases, and I'm sure they won't withdraw the initiative."
swissinfo, Jonathan Summerton
The people's initiative was launched by a group of victims and families of victims.
They collected over 194,000 signatures - almost double the number required to submit an initiative.
The initiative calls for a single assessment of the risk an offender presents to society.
Parliament and the government have both rejected the proposals contained within the initiative.