The Senate is heading toward showdowns on a series of key Clinton administration foreign-policy initiatives, with the centerpiece - enlargement of NATO - expected to be ratified.

The Senate will take up the measure to add three former Soviet satellites - Poland, Hungary and the Czech Republic - to the half-century-old military alignment next week, and those on both sides are predicting it will receive the two-thirds margin necessary for passage.Even so, opponents of the expansion - a politically diverse coalition - are suggesting they may have picked up some support as more senators focus on the possible dangers of expanding NATO to Russian's doorstep.

The legislation would amend the 1949 treaty setting up NATO, requiring a two-thirds majority in the Senate, or 67 votes. But sponsors contended they could muster that, and opponents suggested they were shy of the 34 votes needed to block the expansion.

The Senate set aside six hours for Friday to debate a House-passed measure authorizing an array of State Department programs as well as an $800 million installment on back U.N. dues.