HOUSTON -- Any doubt that Gov. Michael Dukakis of Massachusetts had just shown he could play in the big leagues was removed by Rep. Richard Gephardt's press-room visit following the Democratic presidential ''debate.''

Like the other six hopefuls, Gephardt was loath to shatter the aura of consensus during the low-keyed two hours over national television. But afterward, Gephardt volunteered to any reporter who would listen that Dukakis was traveling the wrong road toward free trade and higher taxes.

That confirmed the consensus here that Dukakis had starred and Sen. Joseph Biden had bombed in the inaugural of innumerable quasi-debates. Possibly of more permanent interest, it suggested that taxes and trade divide the Democratic Party.

Not much else does. The Houston debate's passive quality derived partly from the absence of any dissenter from conventional Democratic wisdom. Far from containing a George Wallace, the field does not include a Scoop Jackson or even a Sam Nunn who might break ranks from uniform opposition to the contras and the Strategic Defense Initiative. The Rev. Jesse Jackson has become so domesticated that no far-left viewpoint emerges.

Without apparent issue disagreements, pre-debate speculation assumed a Gephardt-Biden shoot-out centering around the nomination to the Supreme Court that day of Robert Bork. Cheered by Biden's long-awaited rise in the polls, his advisers were elated by the prospect of protracted national exposure presiding over the confirmation hearings as Judiciary Committee chairman.

Gephardt operatives seemed just as happy, pointing to Biden's contradictory comments about Bork. In response, the Biden camp called Gephardt's opposition to Bork superficial, trying to exploit 14-year-old Watergate memories.

No hint of this was revealed in the debate; Bork's nomination was never mentioned. But advice to Biden from his handlers that he must not repeat his notorious tirade against Secretary of State George Shultz may have handicapped him here. He seemed most ill at ease of the seven.

When Biden ventured a polite swipe at the protectionist Gephardt Amendment, it fell so flat that Gephardt did not feel constrained to reply. The trade issue was effectively raised not by a grinning, seemingly flippant Biden but by the earnest, forceful Dukakis. The governor flatly opposed the oil import tax supported by Gephardt and the other candidates.

Serving up the current Democratic dogma that the budget deficit is choking the economy, Dukakis went the next step by practically endorsing a tax increase. Gephardt jumped on it, asserting that the 1986 tax reform should be given a chance (a position echoed only by Biden among other candidates).

These differences were drawn much more sharply after the debate. Gephardt aides, alarmed by Dukakis' vigor, said the governor's revulsion against oil import fees indicted him as a New England regional candidate. As for proposing higher taxes, wasn't this Walter Mondale revisited? ''He even looks like Mondale on television,'' a Gephardt agent told us.

Without going ad hominem, Gephardt said much the same thing in post-debate chats with reporters. On trade, he raised important questions that would have significantly expanded the dimensions of the actual debate.

Gephardt took all his rivals to task for advocating greater productivity and competitiveness in international trade but not addressing the problem of foreign protectionism. ''That's no different from the Reagan administration,'' he said. None of the other six, he added, endorsed the Gephardt Amendment. So Gephardt, while seeming to back away from the trade issue in export-conscious Iowa, sees it as a cutting national issue for him and the Democrats.

If Biden was the only real loser here, there is little doubt he is capable of improving in future debates. The real importance of Houston was what it said about taxes and trade. Should the Democrats take a hard line on trade and a soft line on tax increases, as Gephardt suggests? In the answers may lie the shape and outcome of the 1988 election.