Reagan's Southern sweep points up challenges for both parties

''It used to be your party was like religion: You didn't change very often. Now it's more like choosing a place to eat. People will go with the candidate who looks attractive at the time.''

This comparison, by Univerisity of Georgia political scientist Charles Bullock, goes to the heart of the challenges facing the Republican and Democratic Parties in the South following President Reagan's 49-state reelection victory.

As in three of the last four presidential elections, the Republicans attracted a majority of Southern voters to their ''restaurant,'' but only for the main dish - the choice of president. Most went across the street to the Democratic restaurant for the rest of the meal - US House and Senate members, governors, and local offices.

There were exceptions of course: A small but growing number of Southerners over the past decade or so have been voting a straight Republican ticket.

But does this trend mean a major realignment of voters is under way? Are Democrats moving to the Republican side for the whole slate of candidates? Such a trend in the South could set the stage for a Republican Congress.

Political analysts interviewed give answers ranging from no, to maybe, to yes. But they agree there was no major shifting of Democratic loyalty to total Republican loyalty this time around.

In an 11-state Southern region, from Virginia to Louisiana (not including Texas), Republicans made a net gain of two governorships but only two US House seats, and lost one US Senate seat.

''Republicans have been gradually creeping up,'' says Professor Bullock. ''I think it's going to come (a major realignment), very, very slowy.''

Dan Carter, an Emory University professor of Southern history, agrees: ''In the next 20 years we'll continue to see the Republicanization of the South.'' The Democratic Party in the South will likely become ''a minority party of essentially blacks and 20 to 25 percent of the whites.''

But over the next four years, Republicans are likely to loose ground in the South due to internal divisions over policies, if the economy turns down, says Professor Carter.

Anne Permaloff, another political scientist, says no major realignment has occurred yet. But ''I think it could,'' she says. The associate professor of government at Auburn University's campus in Montgomery, Ala., adds, ''The Republican Party, to build itself into a solid level, has to build at the county and state legislative level. That is beginning to happen.''

Carter says, however, that the Republican Party is most likely to grow in the South by a ''trickle down'' system, that is, from the top down, by having attractive GOP presidential candidates that coax former Democrats to vote a straight Republican ticket.

Southern pollster Clairbourne Darden sees ''a continuing trend toward conservative-to-moderate'' voting in the South and the rest of the nation. But in the South, he says, that does not mean growing Republican strength except for choice of president. Such a trend has ''been starting forever'' and is not likely to go anywhere, he says. One reason is because ''the local Democrats are very close (in ideology) to the national Reublicans'' anyway, so there is not much need for a switch, he suggests.

''The Republican Party is a presidential party in the South,'' says North Carolina political consultant Walter DeVries. ''It's only at the presidential level where they (Democrats) are in bad shape,'' he says. The election of a Republican governor, US senator, and four of seven (a gain of two) US House members this time in North Carolina was probably a ''special case,'' not typical of Republican strength in the South, he says.

There is nothing new about Southern Democrats voting for a Republican president and Democrats for other offices. The South has not been solidly Democratic since 1948. That was the year President Truman lost four Deep South states to break-away Democratic candidate Strom Thurmond, now a GOP senator from South Carolina.

But that Republicans have a long way to go in the South is seen, for example, in several Alabama counties where Democratic candidates for the US House faced no Republican candidates.

The Democrats have problems, too, says Professor Permaloff. ''Their basic problem is they're having to decide what they stand for,'' she says.

One issue that must be faced is how much of a voice blacks will have in the party, she says. Nine out of 10 Southern blacks voted for Walter F. Mondale, according to CBS News/New York Times exit polls.

In Alabama and across the South, blacks have become increasingly active in the Democratic Party, and in voter registration. Black leaders such as Jesse Jackson are claiming blacks saved Democratic candidates from further losses across the South. That may be true.

But, says Permaloff, partly due to black participation, the Democratic Party in the South finds itself more liberal on economic and civil rights issues than many Southerners. ''The party could be moving out of step even more'' if it continues on that path, she says.

In Mississippi, syndicated columnist Bill Minor says the Democratic Party may well become the party of mostly blacks. But Steve Suitts, executive director of the Southern Regional Council, a research organization on issues of poverty and race, says most Southern whites backed President Reagan because they liked his economic policies. It had little to do with antiblack feelings, or feelings that blacks are taking over the Democratic Party, he says.