"It will lead to the weakening of Switzerland," Blocher said. "Freedom and the rights of the people will be limited, and neutrality will at the very least be deeply damaged."
Foreign Minister Joseph Deiss assured voters, "Switzerland remains neutral."
The seven-member governing Cabinet, which had campaigned hard for approval, expressed relief at the outcome.
"Everyone stands to gain from this," a government statement said. "Switzerland will now be better able to safeguard its interests and assume its responsibilities in the world."
Swiss industry and banks had feared a no vote would make Switzerland an international outcast with a selfish and uncaring reputation.
"Today is a great day for Switzerland," said Deiss.
However, President Kaspar Villiger said he was concerned that the cantonal vote was so close and said it indicated a "malaise" that had to be studied and taken seriously.
Switzerland has not moved to join any military alliance, such as the North Atlantic Treaty Organization. But the government does have a longer-term goal of joining the European Union (news - web sites), which is expected to encounter even tougher resistance.
Deiss said the U.N. vote had "nothing to do with entry into the EU."
U.N. Secretary-General Kofi Annan (news - web sites), who also had urged Swiss voters to approve membership, welcomed the vote and said it took the United Nations closer to universality.
The referendum recalled Switzerland's vote to join the United Nations' forerunner, the League of Nations, in 1920. Switzerland became a member, but the league was seen as largely ineffectual — in part because the United States stayed out.
Nationwide, 1,489,062 people voted in favor, compared with 1,237,725 against.
Switzerland has long been a dues-paying member of some U.N. specialized agencies like the World Health Organization (news - web sites) and the International Labor Organization. And recently it has been stepping up its contributions to peacekeeping operations.
The nationalists plastered the country with posters calling U.N. membership a waste of money.
The government says membership should cost $42 million a year, but dismisses that amount as minimal in comparison with the $1.8 billion a year brought to Switzerland annually by the presence of the United Nations' European headquarters in Geneva.