PARIS -- Behind his personal statement Wednesday supporting the new U.S.-Soviet nuclear deal, President Francois Mitterrand is worried that neither the United States nor its NATO allies are anywhere near prepared for what may come next.

Mitterrand, a pro-defense Socialist, believes Soviet leader Mikhail Gorbachev's agreement to change the face of Europe by destroying U.S. and Soviet medium-range missiles is merely the first stage in a ''dynamic'' process. He and his closest advisers worry that as Gorbachev offers other deals to split the West in the name of peace, there is no agreed Western strategy for handling surprise moves by the imaginative Soviet leader.

One possible danger: a Gorbachev call for the removal of all foreign forces on the territory of the two Germanies. Asks a key Mitterrand aide: ''If Gorbachev dared risk such a demand, could the West resist it?"

Mitterrand's own view that the proposed new intermediate-range missile treaty contains more good than bad is not shared throughout the conservative government of Prime Minister Jacques Chirac. Defense minister Andre Giraud privately used the pejorative word ''Munich'' when he first learned last spring that President Reagan and Gorbachev were on the verge of an INF agreement. It is not widely known that Giraud tried to resign from the Cabinet, but was persuaded to stay by Chirac.

Chirac himself has a low regard for the treaty that is destined to be signed at a Reagan-Gorbachev summit between Thanksgiving and Christmas. Anyone who sees it as an unmixed blessing, he has warned, is engaged in self-delusion.

This disagreement within the bifurcated French government is being kept under as tight wraps as possible. In fact, however, it mirrors a similar and debilitating lack of consensus within the entire NATO alliance on the still-unknown implications of the first U.S.-Soviet accord to reduce nuclear arms.

The Elyse'e Palace is cautious about criticizing the United States, particularly Ronald Reagan. But it is to the White House and to Secretary of State George Shultz that French policymakers look for overdue assurance that the new treaty will not denature nuclear weapons as the only deterrent to Soviet aggression. Also badly wanted here is a strong statement by Reagan himself that the removal of the Euromissiles will increase, not reduce, the value of the U.S. pledge to keep American troops in Europe.

The problem with these two bedrock hopes is immediately apparent. Reagan's insistence on full speed ahead with the Strategic Defense Initiative is based on what he calls the ''immoral'' and the ''obsolete'' nature of nuclear weapons, either as deterrent to aggression or on the battlefield. Likewise, pledges of enhanced U.S. intentions to keep American troops in West Germany are highly unlikely in the preelection political campaigning now in full swing across America and on Capitol Hill.

''The worst danger,'' one high-level policymaker here told us, ''is political. As long as Reagan really believes nuclear weapons are immoral, and Gorbachev plays that theme back to the West, the nuclear deterrent will lose credibility.'' If he is right, the follow-on negotiations to reduce strategic weapons and possibly short-range tactical nukes will further erode public faith in the nuclear deterrent.

Mitterrand and Chirac are in agreement on the necessity of developing closer military links to West Germany. Even dual-key sharing of the futuristic French air-launched nuclear weapon called ASMP is being quietly talked about here. So are more prosaic steps, such as joint training for French and German officers, a French-German brigade of 5,000 to 10,000 men and possible joint French-British targeting of their respective nuclear arsenals. That prospect would have been impossible before the U.S.-Soviet INF treaty became a reality.

But these objectives are shrouded in uncertainty -- and encumbered by decades of failed efforts. The real need is for the United States to help its weaker allies absorb the shock of the superpowers' appearing to move away from the nuclear deterrent that has successfully protected them for 40 years.