aving left many questions unanswered last week in Boston, George W. Bush and Al Gore have a chance tonight to fill in some of the blanks in their second debate.

Do they have the capacity to discuss issues sensibly - as their running mates did last Thursday - without getting prickly and repetitive? Can they offer thoughtful answers and not just the prepackaged mini-ads, complete with ''ordinary Americans'' brought in to illustrate their points? Can moderator Jim Lehrer make the candidates more responsive, and can he probe their differences more effectively?

The Boston debate did show some difference in philosophy, most clearly on tax policy. Bush's statement that a president shouldn't be manipulating tax policy carried a laissez-faire tone reminiscent of another era.

But the two should be challenged on a far greater range of issues and approaches. With welfare no longer an entitlement for the very poor and little progress toward universal health care, what does America guarantee to each of its citizens apart from a vote? At another level, what steps should the United States take to limit the proliferation of nuclear and biological weapons and their use by terrorists? How should such a terrorist attack be met?

The candidates should also be questioned about some of the dubious positions they share, such as favoring the deployment of an unworkable ballistic missile defense shield and the ridiculous embargo against Cuba.

But Lehrer gave no hint last week that he is inclined to stray outside the box constructed by the two candidates. This is too bad, and it raises anew the question of whether the Presidential Debate Commission was right to limit all three debates to the Republican and Democratic candidates alone. That question was underscored on Tuesday when commission officials barred Green Party candidate Ralph Nader not only from the hall, though he had a valid ticket, but even from the grounds.

This embarrassing, undemocratic action made clear that the commission is very much a bipartisan group, representing the Republican and Democratic parties, rather than a nonpartisan group representing the voters. And the corporate control of the event was also a bit thick - did the commission's eagle need to replicate so closely the logo of one of its lead sponsors, Anheuser-Busch?

The commission's job is not easy. The voting public would not be well served by free-for-all debates in which potential presidents were forced to jostle with single-issue zealots. But the bar for third-party candidates should not be so prohibitive as to exclude them from all the debates under nearly all circumstances.

Tonight, it is up to Bush, Gore, and Lehrer to show that Boston was only a beginning.