President Carter's new warning against dangers of "Eurocommunism" in Italy reflects a confrontation with reality after one year in office - by the President in general and by Ambassador Richard Gardner in particular.

When plucked from his chair as professor of international law at Columbia University to be ambassador in Rome, Gardner joined other new Carter officials in viewing Henry Kissinger's hard line against Western European Communist parties as a cold-war throwback. Just before the first anniversary of the Carter inauguration, Gardner helped guide policy back to the Kissinger line: Communist power-sharing in NATO member nations must be resisted with maximum political pressure.

This shift is only the latest signal that Carter is reverting to more conventional anti-Soviet policies, following the confusing rhetoric early in his administration. No longer is the Soviet-Cuban intervention in Angola rationalized as "stabilizing." A strong U.S. reaction to the Soviet role in Ethiopia is now given top priority.

The rhetorial shift on Eurocommunism, proclaimed in a Jan. 12 State Department statement, typifies the administration's new realism about military and political vacuums. Carter is now aware they will quickly be filled by Soviet penetrations if the United States fails to make the case for the Western democracies publicly and forcefully.

So the Jan. 12 statement was both public and forceful: "We do not believe that the Communists share" the "profound democratic values and interests" of Western political systems. The United States "would like to see Communist influence in any Western European country reduced."

Those assertions and the warning that recent Italian political developments "have increased the level of our concern" are far more pointed than last April's policy statement, which avoided direct attack on the Communists. The still-neophyte Ambassador Gardner helped draft the April 1977 statement; the more seasoned Ambassador Gardner was directly responsible for the January 1978 statement.

The administration's claim that the new policy is a restatement of the old is belied both in the words themselves and in Gardner's profound conviction - the product of one year's experience in Rome - that Soviet influence is pervasive at top levels of the Italian Communist party. He is also convinced that the Soviet Union, helped by the Czechoslovak and East German Communist parties, is the major source of the terrorism that has brought Italy close to anarchy.

Gardner's quick flight to Washington two weeks ago to argue for the new hard-line policy was intended as a warning to the old guard of the Christian Democratic Party not to yield to Communist Party pressure without an all-out fight. Strong factions in the old guard, which has ruled Italy for 30 years, would accept alliance with the Communists to cling to power in a coalition government.

Younger, more progressive Christian Democrats seek another courses a top-to-bottom shakeup of their stratified party and its stale, rigid policies. That is also what Gardner wants, on the basis of his political education in Rome.

To cite one example, a luncheon was arranged for a Nov. 22 visit to Rome by Rep. Peter Rodino (D-N.J.) and Health, Education and Welfare Secretary Joseph Califano, with Gardner invited. The hosts were Christian Democratic members of Parliament. Shortly before the luncheon the U.S. Embassy discovered that two Communists would also attend the luncheon.

That smacked of political sabotage, alarming the embassy and increasing Gardner's concern that some Christian Democrats were intentionally promoting a coalition with the Communists. Without any publicity, the luncheon was canceled.

Such events stage-managed by Christian Democratic politicians are extremely useful to the Communists. Played up in the left-wing press, they are supposed to "signal" subtle changes in U.S. policy by showing Gardner consorting with Communists.

The result has been Gardner's insistence and Carter's agreement on eliminating the ambiguities of the Arpil statement. Such realism is both over-due and welcome. Even if it fails to arrest Italy's political decay and the shattering effect on the Western alliance of Communist power-sharing in Rome, it reveals an awareness of reality by the administration sadly lacking when Jimmy Carter took office a year ago.