Senator McCarthy's success in New Hampshire will deal a serious, perhaps fatal, blow to the radical anti-war movement at Harvard and across the country.

Radical student leaders here believe that McCarthy or Kennedy will draw off enough campaign support from previously uncommitted students to cripple such projects as the Harvard Draft Union.

These uncommitted students have been leaning toward a complete break with the political system in recent months.

The latest draft policy decision appeared to be the final shove--pushing the students into the waiting arms of the radical movement, which has been trying for years to build a political base among students.

But McCarthy's success will change all that. His campaign appears as a "last chance" for American liberalism.

Instead of meeting their friends in Chicago for a roaring anti-war demonstration outside the Convention hall, they will be inside. As McCarthy said Tuesday night: "If we go to the convention with this kind of strength, there will be no riots. There will be no demonstrations, but a great victory celebration."

Radicals here are jealous of McCarthy's quick success in harnessing anti-war support--although they are quick to point out that their protests laid the groundwork for his achievements.

The radicals object to McCarthy on three grounds:

* He is a product of the same political system that produced President Johnson. If elected, he will be subject to the same pressures from the military and big business and will have to act in similar ways.

* His stand on foreign policy is far different from the radicals'. He does not believe that U.S. business and military involvement in the "third world" is wrong. And even in Vietnam, he does not favor immediate unconditional withdrawal.

* Like all liberals, he is merely holding up an inevitable confrontation between the established order and "progressive" forces in society.

In the long view, however, many radicals believe that the McCarthy campaign could serve as a valuable purpose for them.

They are confident that the Senator will lose the Democratic nomination to Johnson and so "prove" that, even with popular support, an anti-war candidate cannot beat the system.