Trailing slightly in the polls and substantially in estimates of his Electoral College strength, Michael Dukakis needed to hit a home run Thursday night in his debate with George Bush.

Despite a generally able performance, Dukakis did not do so.As a result, the last of the 1988 debates merely confirmed previous impressions of the two presidential candidates.

Though somewhat warmer and more relaxed than during their previous televised confrontation, both still came across as basically decent, competent, likable men - with sharply different ideas about what's best for America.

The impression might have been somewhat different if the format of their appearance had been different, if Bush and Dukakis had questioned each other instead of merely participating in what amounts to a joint news conference.

Or the impression might have been somewhat different if the questions from the panel of reporters, though generally quite pointed, had not veered quite so often toward the inane.

Questions, for example, like: Can't the Republican and Democratic presidential contenders find something nice to say about each other? They could - very briefly. Can you identify some current role models for Americans? Guess how many votes, if any, the responses changed. Must the President of the United States be likable to be effective? Of course not. What can be done to make the current campaign less negative? Bush ought to stop pinning the "liberal" label on Dukakis, in Dukakis' estimation. And the press ought to look for positive things to report, in Bush's estimation. What about political corruption? Neither party has a monopoly on it. So what else is new?

Where, then, does all this leave the presidential campaign? Pretty much where it was before Thursday night's debate.

When the party in power has peace and prosperity plus a popular incumbent in the White House, its nominee is hard to beat. Bush just needed to reassure those who already have decided to vote for him. Dukakis needed to persuade voters it's time to change.

In trying to make his case, Dukakis repeatedly referred to Bush's judgment as reflected in his selection of Dan Quayle as the GOP vice presidential and to Quayle's performance in the vice presidential debate with Lloyd Bentsen. Dukakis might have been better advised not to have pressed the point in view of polls showing that Bush was not damaged by Quayle's performance in last week's debate. Indeed, though Bentsen is generally conceded to have had the edge in that debate, his performance prompted continuing comment that the wrong Democrat is at the head of the ticket.

Though neither candidate made a costly blunder during Thursday night's debate, it's still too soon for either to take victory or defeat for granted. Surveys showing Bush ahead also show that much of his support is soft and in some states he holds a very narrow lead. Harshly negative assaults could still backfire. If the momentum and the big states start moving toward Dukakis, he could still win.

After the debates, then, things look good for George Bush. But nothing will be in the bag until Nov. 9.