A closer look at Congress's close vote on aid to El Salvador the other day is instructive. Mr. Reagan's winning 212-to-208 margin came in the South, which gave him a 50 -vote edge. The other three regions voted against the aid, by nearly 10 votes in the West and Midwest and nearly 30 votes in the East.
This is a predictable enough pattern. The South tends more readily to support arms, to rally to anti-Cuban, anti-Soviet themes. Not all the South went along. North Carolina, which politically seems to be acting more Northern all the time, voted against it, as did more liberal Maryland and West Virginia. But the majorities of the other dozen Southern delegations voted with President Reagan. Or, some would argue, with House majority leader Jim Wright, the Texas Democrat who pulled crucial moderate Democratic votes into the pro-aid camp.
In much of the rest of the country - including the Southwest, which like Texas has large Hispanic populations - the congressional vote pretty much followed party lines. In California, Minnesota, Illinois, Michigan, Indiana, and Ohio total party conformity prevailed.
This mixture of votes, some swayed by the merits of an issue and others held fast by partisan inertia, suggests how complex congressional decisions can be. ''Win'' and ''loss'' can be oversimplifications. That's why Congress considers the same issue over and over in various stages of the legislative process and in many guises.
The White House admits that President Reagan swayed few if any votes in his televised address on Central America on the eve of last week's vote. It would have taken several days at least for any public reaction to have an impact on Congress. The Democrats say their counts of votes before and after the speech were the same. The shape of the vote was well set. Then why the speech? Central America appears to be a vulnerable flank for the administration for this fall's election. The South is crucial to Reagan, and the speech enabled him to stay in touch with followers who agree with his foreign assistance, East-West approach. A speech followed by a congressional victory, even an assured victory, reinforces his Washington leadership image. Note that the White House decided to go to the public on Central America and not on the MX missile issue this week, where Capitol Hill leaders projected a White House setback.
President Reagan told visiting Mexican President Miguel de la Madrid Tuesday that the US cannot close its eyes to an impending flare-up in Central America. Mr. de la Madrid countered by warning against interventionist US maneuvers that risk generalized war in the region. At other times the debate over US policy is described as a choice between diplomacy and force, of economic vs. military aid.
What the House vote on El Salvador showed is that such arguments are not settled in Washington in up and down ballots on their merits. Public debate - from the Oval Office and on the Capitol Hill floor - must compete to dislodge congressmen from broader ideological, regional, and partisan patterns. Such patterns change slowly, over years, election to election, seldom overnight. The next shift in the US's Central American alignment will likely come from events - progress of a new government, a call for American troops - rather than from debate.