FOR ALL THE yawning over the scripted, infomercial nature of the modern nominating convention, the Democratic National Convention getting underway in Boston today represents an important opportunity for John F. Kerry. The bad-mouthing of conventions is amply justified; we've engaged in some ourselves. And yet, even the 21st-century convention -- devoid of suspense, stripped of even polite disagreement, relegated to a mere hour nightly on the networks -- serves a significant purpose, and we don't mean the chance to scoop up the goody bags that convention partygoers seem to grab as greedily as 5-year-olds at a birthday party. Scripted or not, the convention offers Mr. Kerry's biggest chance yet to explain to the American people why they should elect him president, to address their concerns and uncertainties about his candidacy, and to explain -- more crisply than he has so far -- where a Kerry presidency would take the country.

In some ways, the stakes for Mr. Kerry are not as high as for many nominees. In previous years -- think Bill Clinton in 1992 or George W. Bush in 2000 -- the convention has served as a launching pad to introduce the nominee to a nation that scarcely knows him. This year, though, the electorate is so engaged and polarized, and the sliver of undecided voters so meager, that Mr. Kerry needs something less in the way of an introduction. Most voters have already decided whether they will support him.

In other ways, though, Mr. Kerry's task is daunting: He needs to attract the attention, and assuage the doubts, of the voters who are most likely to be channel-surfing through his acceptance speech. A new poll from the Pew Research Center for the People and the Press underscores the magnitude of the challenge. More than half of those surveyed said they planned to watch little (29 percent) or none (25 percent) of the coverage. But while such voters may not be able to explain the details of Mr. Kerry's health care plan, the convention coverage, seeping through the filter of late-night monologues or quickly scanned headlines, can nonetheless help plant a positive image of Mr. Kerry in the minds of these voters.

Many potential voters are leery of rehiring President Bush, but not sold on Mr. Kerry either. Indeed, the Pew poll indicates that enthusiasm for Mr. Kerry is lagging behind support for his party. Democrats enjoy a strong advantage on the economy and other domestic issues and are competitive with Republicans on foreign policy matters with the exception of fighting terrorism. Nonetheless, as the Pew researchers put it, "For all that, there are no signs that Kerry is breaking out in the presidential horse race."

Mr. Kerry hopes to use the convention to do that; we hope he might depart from his script so far to do even more. Yes, he would eliminate tax breaks for the richest Americans in order to expand health care coverage and improve educational opportunities. But what about tackling the looming costs of Social Security and Medicare as the baby boomers quickly approach retirement age -- a subject on which both campaigns have been notably silent? The deficit that Mr. Kerry would inherit as president is in significant part the legacy of the imprudent policies of the incumbent administration, but Mr. Kerry has not leveled with the American people about the inevitable pain of extricating the country from this fiscal hole.

Similarly, on the foreign policy front, Mr. Kerry commendably promises to repair frayed international relationships while staying the course in Iraq. But Mr. Kerry can talk all he wants about the desirability of persuading allies to share the burden; the reality is that few more troops are available to be sent to Iraq or Afghanistan even if allies could be persuaded to do so. And so, as with the deficit, the course will be harder than Mr. Kerry has so far acknowledged. A little straight talk from Boston would be a welcome, if unlikely, addition to the presidential race.