WASHINGTON (AP) _ The campaigns of both Jesse Jackson and Pat Robertson face a critical question as they move into the Super Tuesday contests - whether they have been able to expand their traditional bases of support.

Both need to attract more than protest votes from those who want to ''send a message'' to the more mainstream candidates.

While Jackson has shown he can attract substantial votes outside the black community, analysts still question whether Robertson can move beyond his fundamentalist Christian base.

A crucial test for Robertson takes place Saturday when South Carolina Republicans vote in a presidential primary in which he once said he would beat George Bush and Bob Dole convincingly.

''If I lose this one, then I'm in trouble,'' Robertson said of South Carolina after his fifth-place finish in New Hampshire.

The South Carolina contest comes three days before Super Tuesday, the March 8 showdown when 20 states, concentrated in the South, hold primaries or caucuses.

Jackson and Robertson both have strong bases in the region, but each needs to reach beyond his core constituency.

Robertson startled many Republicans with a strong second-place finish in Iowa and was second again in South Dakota and Minnesota.

Opinion polls in South Carolina show the former television evangelist running third.

Jackson also was second in Minnesota and followed that with similar showings in Maine and Vermont - all states with very small minority populations. In each state, the winner was Gov. Michael Dukakis of Massachusetts.

Ann Lewis, a political adviser to Jackson, argues that he has ''gone from being a protest candidate to being a message candidate.'' Ms. Lewis said Jackson's message this campaign is far different from what he was saying in 1984.

''The last time he talked a lot about the party process and being locked out,'' she said. ''His message this time is economic and inclusive.''

Pollster Peter Hart said he believes Jackson has moved to what he called ''the polarizing mainstream.''

''To suggest he is just like (Rep. Richard) Gephardt or Dukakis is probably incorrect,'' said Hart.

The pollster said that his surveys show that about 85 out of 100 Democrats would consider voting for Gephardt or Dukakis, but the number drops to 60 out of 100 when asked about Jackson.

Hart said the mix he has found among white supporters of Jackson is people who are younger, more liberal and in some cases economically dispossessed.

Republican pollster Lance Tarrance, who has been working for Rep. Jack Kemp, said his surveys in South Carolina and elsewhere in the South find ''Pat Robertson hasn't quite created the brush fire we thought.''

He said Robertson needed to get support from the ''non-establishment vote - we don't have anti-establishment people in the Republican Party.''

So far, he said, that hasn't happened. One reason he cited was that Bush ''has held together a lot stronger than we had hoped.''

Analyst William Schneider of the American Enterprise Institute, a Washington think tank, said Jackson's white support ''tends to be the radical- chic votes'' - affluent young liberals.

He said those are voters who are turned on by what he called ''the Democratic revival speech,'' a statement of traditional party values.

''Jesse Jackson is the only one in the race now who can give the Democratic revival speech,'' said Schneider. He cited Sen. Edward M. Kennedy of Massachusetts and Gov. Mario Cuomo of New York as non-candidates who deliver the same sort of speech.

Schneider said that while Gephardt appeals to blue-collar voters ''he's not selling himself as a liberal.'' He said Gephardt also is hurt because people who still want to hear that appeal demand authenticity.

''Fundamantalist believers (in that message) don't flip-flop,'' said Schneider.

As for Robertson, Schneider said, ''The base of his support is the limit of his support.''

Political scientist Norman Ornstein of AEI, said he is ''skeptical that this is some dramatic expansion of Jackson's base.''

He said the Vermont and Maine results were examples of contests where there was ''a low turnout with a real emphasis on the issue activists.''

''Jackson's positions are going to be very attractive to issue activists whether they are white or black,'' said Ornstein.