WHEN A PARTY'S national chairman speaks on foreign policy, you expect a partisan view. What Democratic National Chairman Charles Manatt's speech last week at Georgetown University showed, however, is not how wide but how narrow are the differences between the two major parties' positions on key arms control issues.

Mr. Manatt argued strongly, for example, that arms control negotiations should not be suspended in retaliation for the Soviets' shooting down of the Korean jetliner. This is President Reagan's position as well--and the right one. The Democrats' quarrel here is with the Richard Vigueries of the right, not with the president. Mr. Manatt says that SALT II should be ratified and, pending that, its terms observed. The Republican president, though he campaigned against it, is observing it now. It was a Democratic president and Congress that couldn't get it ratified in 1979 and 1980.

True, Mr. Manatt, professing to speak for the seven Democratic presidential candidates, repeated the endorsement of a "mutual and verifiable" nuclear freeze that his party made at its Philadelphia mid-term conference in June 1982--although one candidate, Reubin Askew, opposes the freeze. But the freeze appears to be yesterday's issue. Many of its backers admit it is symbolic and intended chiefly to put pressure on the Reagan administration.

Mr. Manatt pledges that the Democrats will seek "prudent, balanced, verifiable" arms control negotiations. The Reagan administration pledges itself to the same goal. Mr. Manatt suggests that there are differences between their approaches. But it's hard to say precisely what they are. The Democrats, while suggesting they are more serious about arms control, do not wish to be seen as soft on Soviet power. Mr. Reagan, since he got engaged in negotiations, has been moving in a way to preempt some of the Democrats' ostensibly warmer concern for arms control.

Mr. Manatt was at pains to explain that the Democrats understand the Soviets to be a nasty bunch, "rigid, militaristic, paranoid and hostile to our most sacred values." Is that so remote from the "evil empire" some Democrats were tut-tutting President Reagan about not so long ago? There are differences, and some important ones, between the parties. But on arms control, they may have more in common than they sometimes want the public to believe.