HOUSTON -- Although George Bush won handily in the GOP presidential gabfest here, mainly by not losing, the real news was a rugged display of vitality by post-Reagan Republican leaders, who showed the capacity to excite at a time when their administration is near death's door.

Compared with the passivity if not boredom of Bill Buckley's Democratic debate in July, the Republicans showed a passion for political differentiation and an ability to express it that dismayed honest Democrats. In their debate, held long before Sen. Albert Gore's precocious walk into unfamiliar party pastures, there was hardly a dissent from wall-to-wall liberal orthodoxy.

The political event here was major in proving that front-runner Bush could easily exceed low expectations (and put some hair on his chest). But it also warned Bush that he cannot escape confrontation and contentious debate about serious issues for the 1988 presidential election: tax hikes, the INF treaty, abortion and others.

Attention centered on the vice president and Senate Republican leader Robert Dole, and somewhat less so on Rep. Jack Kemp. But the highly regarded post-debate SRI poll of 800 voters in Iowa, New Hampshire and the March 8 Super Tuesday primary states turned up a couple of surprises: that Pat Robertson, the erstwhile televangelist, made the ''best impression,'' followed by Kemp; that Kemp did best of all in explaining economic issues, which may well be dominant next year.

Add this certainty: Dole will never again allow himself to be talked into being the caricature of a laid-back, wisecracking good ol' boy. That incognito, intended to conceal the flashing and notorious Dole stiletto, instead buried him in platitudinous muck and one-liners designed for innocent laughter, not political points. He was ''comfortable about himself,'' top campaign adviser and former labor secretary Bill Brock told reporters after the debate. But few viewers felt all that comfortable with the new Bob Dole.

Next time, the combative spirit of antagonism that marked the dialogue here will make Bob Dole be Bob Dole. For if he is to overtake Bush, he must show his mastery in dominating a debate, not as a cutthroat or moderator, but purveying the skilled, experienced leadership he claims is his strong point.

Dole had come a long way in the weeks leading up to Houston. Proof of that was the take of two fund-raisers the next day in California: more than a million dollars at the Century Plaza in Los Angeles and nearly $600,000 the same day in San Diego. Another twin-bill fund-raiser here in Texas in two weeks is expected to raise half a million more.

That shows a campaign with potential, and Dole's expectations are running high. ''It's out there,'' he told a close friend last week, a conclusion he began to take seriously in mid-August. But his overly cautious performance here may have moved the ''out there'' farther out, not closer.

The cool ease with which Bush defused attacks on him from Alexander Haig and Pete du Pont, and then defended his lonely support of Reagan's new nuclear-disarmament treaty, was surprising in view of all the rhetorical gaffes that have tumbled from his lips the past few months. But a kindly warning sent to him the morning after the debate from a member of his California high command needs to be heeded.

The advice: Bush should spell out his education plan (he has said he wants to be known as the ''education president") and his deficit-reduction plan because so far he is not getting any ''converts'' or persuading any fence sitters.

Bush aides say he is well aware of the danger of falling into the 1980 trap of boasting about ''Big Mo'' and has reacted soberly to his win here. He continues to pick up new backers, the latest being Rich Williamson, a conservative with well-established credentials in the Republican right wing. He held several big jobs in the Reagan administration and is now a Chicago lawyer.

Williamson will be the go-between on ideological issues for Bush and the right, which has not yet jelled behind any candidate, although Kemp is close to being its ideological soulmate. The upstate New Yorker is the nearest thing there is in the Republican field to Reagan's clone on strategic defense, right-to-life and taxes.

Thus Williamson's hookup with Bush improves his front-runner position. The assignment is to try to persuade Reaganite conservatives that way down deep Bush is not at all the mainstream, country-club Republican they take him to be.