ust short of his first 100 days in office, the honeymoon appears to be over between Spain's Prime Minister Felipe Gonzalez and the conservative opposition to his Socialist government.

The opposition has focused on the Cabinet decision late last month to take over Spain's largest private conglomerate, Rumasa, and charged that the move indicates that the Socialists are breaking electoral promises to eschew nationalizations and protect private enterprise.

In an opening legal salvo, the opposition has filed an action with the constitutional court on grounds that the government has violated the constitution's safeguards of private property in the Rumasa case. In a more general attack, the opposition claims the seizure will worsen the overall state of the economy.

"The government has undermined investment in the private sector and created a climate of economic uncertainty," claimed one opposition spokesman in parliament.

The government insists there has been no overall change in the Socialists' professed economic moderation and their explicit opposition to nationalizations. It alleges that the conglomerate, which last year accounted for 1.8 percent of Spain's gross domestic product, was near bankruptcy and that serious financial malpractices were involved. The takeover was necessary, government spokesmen said, to protect individual shareholders, depositors in creditor banks and the jobs of Rumasa employes.

But conservative and business opposition to the seizure has fueled a growing polarization of public opinion over Rumasa. The nation's two leading business lobbies, the bankers association and the employers confederation, have called on the government to honor its initial suggestions that the takeover was only temporary.

Since Gonzalez was sworn in as prime minister on Dec. 2, his government has tried to project an image of firm leadership, determined to inject morality into public life. It apparently was hoped that the decision to take over Rumasa would demonstrate decisiveness, and the allegations of fraud, negligence and malpractice leveled against the group are in line with the government's clean-up orientation.

Public concern is increasingly focusing, however, more on Gonzalez's election promises to create 800,000 jobs during his four-year mandate and "put Spain back to work." Unemployment stands at 17 percent, the highest rate in Western Europe, with 2.1 million jobless.

Following the Rumasa takeover, business circles noted a long list of potential economic upheavals ahead. Chief among them are a deadline at the end of this month for the restructuring of Spain's major petrochemicals group, Explosivos Rio Tinto, which in September negotiated a six-month moratorium on repayment of $1 billion of debts. A July 1 date also has been set for agreement on restructuring Spain's steel sector, which is losing money.

The Rumasa conglomerate is something of a special case in Spain because of its size. Its hundreds of directly controlled and associate companies employ an estimated 60,000 persons and are involved in sectors ranging from agriculture and wine exports to shipping and real estate. Rumasa is a household name to Spaniards, and its bee logo is as familiar as a bullfight poster.

According to senior economics minister Miguel Goyer, who holds the portfolios of treasury and commerce, a formal expropriation was necessary--rather than a low-profile government intervention--because of the size and complexity of Rumasa's holdings.

The shock over the Rumasa takeover Feb. 24 was compounded in succeeding days as government officials disclosed figures showing that shares in the group's directly controlled 18 banks were grossly overvalued. In addition, monetary authorities estimate that the group owes the treasury close to $500 million in taxes and social security contributions.

Business sources said that there was growing apprehension over the ability of the government to turn around the Rumasa group even with a massive injection of public funds. Officials investigating the conglomerate concede that some Rumasa companies are unlikely to return to the private sector because they will not be financially viable.