PARIS, DEC. 16 -- This weekend's decision by the 12-nation European summit to move quickly toward political and economic unity generated a wave of euphoria, but the buoyant mood is likely to fade soon as governments confront the domestic consequences of their ambitious plans.

The twin conferences launched in Rome to negotiate the details of European unity are expected to conclude late next year with detailed outlines for achieving a single European currency and a common security policy in this decade. But already there are fresh signs of hesitation in several European Community states as voters and politicians contemplate the impact of surrendering control over policies on currency, foreign affairs and defense.

Britain has long expressed deep reservations about parting with its cherished sovereignty. During her tenure as prime minister, Margaret Thatcher's disdain for European federalists and stubborn refusal to cede power to supranational bodies highlighted Britain's isolation from its 11 EC partners, which tended to make decisions working toward European unity and ignore the inevitable Thatcher veto.

Under new Prime Minister John Major, the suspicious attitudes toward Europe are slowly being replaced by a more cooperative spirit and less polemical tone that is winning the appreciation of his European peers.

But the suspicious attitudes have not vanished. Many ardent supporters of European unity are convinced that London's close links with the United States will always compromise its loyalty to Europe.

What worries some of them, such as EC Commission President Jacques Delors, is that Major will be more successful than Thatcher in finding allies to block the difficult decisions that lie ahead in giving up vital parts of national sovereignty.

"We could always work around Thatcher, because her style was so unsympathetic that other European leaders never wanted to be seen on her side," observed a senior EC official. "But Major may be able to rally other leaders to join him in stopping moves toward a single currency or defense policy."

French President Francois Mitterrand and German Chancellor Helmut Kohl have vowed to push jointly for a European security plan to be conceived over the coming year as part of the blueprint for political union. Each has an uncontested grip on power and will not face any important elections before the plans for European unity are submitted to national legislatures for ratification by the end of 1992.

While their secure leadership positions would suggest a freedom to back bold initiatives, both Mitterrand and Kohl are uncomfortable about risking a domestic backlash against Europe, according to French and German officials.

In Germany, the notion of sacrificing the strong, stable German mark for an untested European-wide currency worries much of the population, including the powerful head of the Bundesbank, Karl Otto Poehl.

In France, the thought of surrendering the proud, independent military command, including the national nuclear deterrent that has backed French claims of big-power status since the days of Charles de Gaulle, still arouses volatile political emotions.

Despite recent opinion surveys that indicate 70 percent of all EC citizens and two-thirds of the French public favor a common European defense, French political leaders in the mainstream parties still have been reluctant to make a quantum leap toward European unity. Neo-Gaullist leader Jacques Chirac bluntly rejected the vision of a united Europe two weeks ago, and influential members of Mitterrand's Socialist Party have been withholding support for their leader's bolder pan-European initiatives.

Last week, Alfred Dregger, the legislative leader of Kohl's Christian Democratic party, declared that a European monetary union would be possible only if France abandons its ambivalent approach to security issues and fully accepts the idea of a common European defense. "How can Germany renounce its sovereignty over the mark if its security needs are not satisfied through a European defense policy that must include France?" Dregger said. "Monetary union and security union have equal importance and depend on each other."

The prospect of an imminent global recession also is a cause for concern among the European federalists. They fear that an economic crisis, perhaps triggered by soaring oil prices as a result of the Persian Gulf crisis, could prompt governments to forsake the pan-European policies that flourished in times of prosperity and instead favor special subsidies and other measures to keep their own economies afloat.

As the 12 EC member states strive to consolidate their economic and political union, they also will face increasingly assertive demands in the coming years from non-member European countries that want to join. Last week, Sweden's parliament voted to apply for EC membership, joining earlier applicants Austria and Turkey. In the near future, Hungary, Poland and Czechoslovakia are expected to seek membership or at least some special association with the EC.

Delors and others have warned that acceptance of new members, especially poor and populous countries such as Turkey or Poland, could derail plans for unity within the decade. The EC would be forced to lower its political ambitions to a broad confederation that retains strong powers for each nation-state. While that scenario would disappoint the federalists, it would probably suit the vision of Britain's conservative government.