A presidential decision has been made for a summer summit here with Soviet President Leonid Brezhnev to sign a new strategic arms limitation agreement, a gamble risking Jimmy Carter's leadership at home as well as his command of the West against Moscow's world offensive.

The gamble and risk stem from President Carter's private acknowledgment that he does not now have the votes in the Senate to approve the new treaty. Nevertheless, he is determined to send the treaty to the Senate late this year, without asking for immediate action. He would then seek to mobilize public fear of a nuclear holocaust to build pressure for SALT II in 1979.

What's more, despite increasingly adventurous worldwide conduct by the Kremlin, Carter has reinforced his policy against any linkage of SALT with Soviet good behavior. Moscow's refusal to reduce its pressure on the Horn of Africa is not relevant to SALT. Nor was last week's Soviet-supported government overthrow in Afghanistan, an intervention officially ignored by the U.S. government.

This anti-linkage, turn-the-other-cheek policy raises some eyebrows in Western Europe. But what really builds fear there is the prospect that Senate refusal to ratify SALT II would grievously compromise Carter as a leader of the Western alliance. Such warnings from this country's European allies are being ignored by the president.

The carefully constructed buildup for the summer summit predates the president's March 17 speech at Wake Forest University. It is now clear that the Wake Forest speech, widely interpreted as a policy switch warning Moscow to mend its ways in Africa, was intended for domestic consumption.

Actually, the crucial Moscow SALT session in mid-April was privately arranged between Secretary of State Cyrus Vance and Soviet Ambassador Anatoly Dobrynin just before the Wake Forest speech. Special efforts made certain the Russians understood that Carter was by no means forging a new hard-line policy. Marshall Shulman, Vance's Soviet adviser, alerted Dobrynin not to be misled by the Wake Forest address.

Although Vance has said little about his April talks in Moscow, some SALT II disputes were settled there. Except for a few remaining unresolved issues, the agreement is set in concrete.

Acute alarm is spreading through the Western alliance over signing a treaty that, in Carter's own private opinion, today does not command the necessary two-thirds support in the Senate. A ranking European diplomat told us that failure of the U.S.-Soviet SALT II negotiations would be merely a "setback for detente"; but a treaty defeated in the Senate, he added, "would destroy European credibility in the 'imperial' responsibilities of America."

"Credibility" of U.S. leadership is already in a state of disarray, partly because of Carter's much-criticized vacillation on the neutron warhead and his decision - without getting any Soviet tradeoff - not to put it into production. That is added to previous concern over Carter's unilateral cancellation of the B1 bomber, again without responding Soviet concessions.

Now comes Carter, full speed ahead on SALT, despite the most intensive and successful Soviet offensive since October 1962, when Moscow placed nuclear missiles in Cuba. This timing, following the B1 and neutron warhead cancellations, is the source of serious concern in European capitals.

The ambassador of one of this country's most important non-European allies put it this way to us: "The president's refusal to link SALT with Soviet actions in Africa and now in Afghanistan is the answer to a maiden's prayer. There must be rejoicing in Moscow."

That strong sentiment is now being expressed openly here at home by Henry Kissinger and other former high officials. Citing a "presumption" that the United States may now be facing "a global geopolitical challenge" from Moscow that cannot be reconciled to "any definition of detente," Kissinger warned in a New York talk April 19 that Senate approval of a new arms-control treaty was now questionable.

Kissinger's warning defines Carter's political risk in charging ahead with SALT II in the face of the president's own gloomy appraisal of chances in the Senate. Without the support of Kissinger, which he had in full measure during his nip-and-tuck battle over the Panama Canal treaties, Carter could not count on even a small percentage of the Republican senators who made Panama ratification possible.

Still, the president has made his irrevocable decision to bring Brezhnev here, then take his chances by appealing to the country over the Senate's head. That is a double-or-nothing bet that, if lost, would be the most severe setback yet to Carter's presidency.