Mr. Brokaw put in a bravura performance. Rather than ask everyone the same question — which leaves each candidate answering like “a trained seal,” as Newt Gingrich aptly put it recently — Mr. Brokaw asked questions that were appropriate to the various candidates, so that the debate took on the aspect of an extended and well-informed conversation. He questioned Democrats and Republicans separately, during four 30-minute segments, but the format allowed candidates of one party to criticize those of the other party, which several did.

In the process viewers heard the talking points and propaganda of not just one party but of two, and the prospect that the other side would soon get its turn to speak may have deterred candidates from the kind of cheap shots which would evoke cheers from a one-party audience. My impression, which was reinforced by reading the transcript this week, is that there were fewer of these than in the debates I’ve watched this year. And viewers got more of a preview of the arguments that would be made by either side once the nominees were determined, along with some basis to measure the candidates of one party against those of the other.

Why haven’t there been more such two-party debates during the primary season? In 1992, 1996 and 2004 incumbent presidents were running for re-election, and presumably they would have rejected invitations to debate a bevy of candidates, some of them arguably unserious, from the opposition party. In 2000, there were only two serious candidates for the Democratic nomination, Al Gore and Bill Bradley, who would have been heavily outnumbered by Republicans on a two-party stage.

But this year there are roughly equal numbers of candidates in both parties. A two-party debate at this point might be cumbersome: eight Democrats and nine Republicans have been participating in one-party debates, and even in a two-hour confrontation (about as long as viewers can be expected to watch these days) each of 17 candidates would get to speak for only five or six minutes. That’s a problem that could perhaps be surmounted by holding two debates, one on foreign policy and one on domestic issues.

A better opportunity may come when the early caucuses and primaries winnow the fields substantially and indicate clearly which candidates have only negligible support. The January primary and caucus calendar — not yet entirely set in stone — will be crowded, but there may be room for a two-party debate before the parties’ nominations are determined.