If the European Common Market isn't careful, it could soon turn a warm friend into a powerful enemy. That's what could happen if the Common Market keeps fending off efforts by Turkey, an associate member since 1963, to become a full member.

The market is running out of excuses. No longer can the Common Market count on Greece to veto Turkey's bid for full membership. Under Prime Minister Turgut Ozal, Turkey has made great strides in recent years in moderating the long-standing hostility between Ankara and Athens.Nor does it make much sense for the Common Market to keep insisting that Turkey is too poor to be granted full membership when such membership would give the Turks access to western markets and the opportunity for more economic progress.

Indeed, as The Wall Street Journal recently reported, Turkey already has made large strides not only toward overcoming its poverty and unemployment, but also toward becoming more democratic and sensitive to human rights.

Consider, then, the possible consequences of rejecting Turkey's bid. Rejection would risk alienating an ally that stands as a barrier to Soviet expansion in the Mediterranean and that ranks only behind the United States in the number of troops it provides to NATO.

A Turkey spurned by the Common Market could easily become more vulnerable to persistent efforts by not only the Communists but also by Islamic radicals to woo Ankara away from the West.

The Common Market needs to do more than just work to build a totally free European market, as important as that is. The market also should strive to make Europe less vulnerable by making it less politically fragmented.

Surely the sensible course is to set a series of reasonable economic and political goals for Turkey to meet and a timetable for its eventual admission to full membership in the Common Market. The challenge now is to stop looking for excuses to keep Turkey out and start seeking ways to get it admitted.