Vice President Bush must show strength and independence. Former Delaware governor Pierre S. (Pete) du Pont IV must exploit the exposure. And former secretary of state Alexander M. Haig Jr. must, in the words of an adviser, "appear relevant to this campaign."

For front-runner Bush, back-runners du Pont and Haig and the three other Republican presidential hopefuls, their first nationally televised debate Wednesday presents a variety of tasks as it holds out the promise of stardom -- and the potential for danger. The debate, a special edition of William F. Buckley Jr.'s "Firing Line" program, will be broadcast live from Houston's 4,000-seat George Brown Convention Center by about 270 public television stations (including Channel 26 {WETA} at 9 p.m.).

Universally acknowledged as a big-league GOP event, it's the first time voters will get a group glimpse of all the Republicans who would be president: Bush, who claims wide support from the party establishment; Senate Minority Leader Robert J. Dole (R-Kan.), who is running a close second in the polls; Christian broadcaster Marion G. (Pat) Robertson, who is well-organized, well-financed and telegenic; Rep. Jack Kemp (R-N.Y.), whose campaign has lagged behind expectations; du Pont, who is trying to make a splash by advocating such programs as farm subsidy phase-outs; and Haig, whose cash-short candidacy is often described as "quixotic."

After Houston, the Republicans will stage a half-dozen more TV debates, but none is likely to attract the interest that attends the political premiere. "In effect, it's the opening game of the political season," said Lee Atwater, Bush's campaign manager. And unlike last July's congenial opening debate of the Democrats -- also a "Firing Line" special aired live from Houston -- Wednesday's spectacle may indeed become a fray as the adversaries stake out their positions on the deficit, the stock market, Soviet summitry and perhaps even each other.

Already Bush has complained publicly about Robertson's and Kemp's campaign tactics, and Haig has called Kemp a "supply-side maniac." Kemp's aides frequently charge that Dole would raise taxes, and Bush has been branded a "lackey" by du Pont, a political "hemophiliac" by Haig, and a "whiny loser" by Robertson. Most of the shots in recent weeks have been aimed at Bush.

"The vice president's opponents have been throwing spitballs at him every time they get the chance," said Atwater. "It'll be interesting to see if they do it themselves at the debate, or leave it to their henchmen and flunkies."

"Everybody's going to try to beat up on Bush," predicted the vice president's media consultant and "chief coach," Roger Ailes, who has worked for a string of GOP presidents and presidential candidates. "We're going to give him some briefing notes and have some meetings," said Ailes, whose campaign heirarchy has decided as a group to keep mum on specifics of debate strategy.

But GOP political consultant John Deardourff said Bush must do better than just good. "With the experience and breadth of background that his people claim for him, it creates the expectation that he will somehow overwhelm the others," said Deardourff, who is not currently working for a presidential candidate but likes Dole's chances for the nomination. "Look at that long resume. Bush can't get by with an average performance. He's got to consistently appear to be better than the others."

Bush, whose formal campaign got off to a shaky start this month with confusion over his stand on taxes (he finally came out unequivocally against raising them) and talk of his supporters' "coming-out parties" (which he blamed for his poor showing in an Iowa straw poll), takes a risk merely by showing up. So does Dole. Both have had unfortunate debate experiences before.

"The only hope the others have under any circumstance is for one or both to stumble so badly that an opening is created," Deardourff said. "The other side of that coin for Bush and Dole is that in the current climate, they are likely to be taken more seriously by the viewers. There's a sort of subconscious selectivity that goes on."

Dole, according to his strategists, is well-positioned to grab the momentum in the two weeks before he formally announces his candidacy, and is affecting a relaxed attitude toward the debate.

His goal, said spokeswoman Katie Boyle, is "to demonstrate his knowledge of issues and leadership, to talk about the need to broaden the base of the Republican Party, and to demonstrate compassion for problems real people are having and to talk about his efforts to find solutions for them."

Dole and Bush are likely targets for their four fellow candidates. "You would expect a certain amount of giant-killing energy," said moderator Buckley, who will be interrogating the debaters along with former Democratic national chairman Robert S. Strauss.

At least one aspiring giant-killer, Kemp, trailing in the polls and heavily in debt, has built a major fund-raising effort around the "critically important presidential debate," as he called it in a piece of direct mail dispatched this month to 150,000 conservative contributors nationwide. Billing himself as "the most conservative candidate running," Kemp urged recipients of the letter to watch the debate and send him $30, $50 or $100, "to make sure I take full advantage of it."

Using the exposure to build name recognition and appear "presidential," Kemp wants to draw a stark contrast between himself and the two front-runners, especially on economic policy, his strategists said. Kemp's advisers were preparing him intensively for the occasion, honing down his rhetoric to manageable brevity and planning debate practice sessions.

The Robertson campaign sees the debate as "a wonderful opportunity for Pat to be clearly identified with issues and leadership ability," said Connie Snapp, Robertson's communications director, noting that it will also be one of his first chances to appear as a candidate on nonreligious television and broaden his appeal.

Du Pont, who has tried to stir interest in his long-shot campaign by taking controversial positions on issues from Social Security to drug testing, plans to attack Bush and Dole on arms control and taxes, and Kemp on Social Security, hoping to get them to talk about du Pont's agenda.

For Haig, the problem is not name recognition, but the perception that his campaign isn't serious. "The people know him as secretary of state, general, NATO commander and chief of staff," said Haig's television consultant Jay Bryant, "but we want them to think of Haig in terms of his candidacy."

As with the Democratic debate last July, Wednesday's two-hour event will allow candidates 90 seconds for their initial replies to questions posed by Buckley and Strauss, and 45 seconds for rebuttals. It also will include brief candidate biographies. Each candidate may pass out 200 tickets to supporters, evenly distributed about the auditorium to prevent formation of cheering sections. Also, as with the Democratic debate, the contenders have been given the first question in advance: What presidential portraits would you hang in the Cabinet Room of the White House?