IN THE CONVENTION that climaxes tonight with the nomination of Sen. John F. Kerry, the Democratic Party has faced two basic challenges. Explaining the evils of the Bush administration is not one of them, for the delegates came to Boston with no doubts on that score. Nor is the normal quadrennial task of unifying the party a burden this year, because the Democrats are as hungry for victory, and as willing to sublimate their many disagreements, as the Republicans were in Philadelphia four years ago. Instead, Democratic leaders are focused, first, on reassuring the nation that it would be as safe or safer under Mr. Kerry's leadership as it is under President Bush's -- and, if they can make that case, on showing that Mr. Kerry would better relieve the economic insecurity that Democrats see as the overarching domestic issue of the election.

Turning to the first task in his masterful Monday night address, former president Bill Clinton described Mr. Kerry as a man of courage ("Send me"), strength and wisdom. Sen. John Edwards, preceded last night by a phalanx of generals who endorsed Mr. Kerry, chimed in with an account of the "decisive, strong" war hero. But only Mr. Kerry can close the sale, and his first objective tonight will be to convince voters that he is a credible potential commander in chief. He has the experience and knowledge of the world to do so; his problem is that his words may be undermined by the gap between his posture and the views of most of those who will nominate him. Mr. Kerry has portrayed himself as a leader who would be more competent and cooperative, but no less resolute, than Mr. Bush in the war on terrorism. But as the convention has made clear, many of his backers are far more dubious of Mr. Bush's assertion that a war is taking place at all, and they flatly oppose his strategy of preemption. Where Mr. Kerry criticizes Mr. Bush's prosecution of the Iraq war, most of his party seems to have no doubt that it was a disastrous blunder in the first place -- skating over the fact that their nominee voted to give the president the power to wage it.

As much as they have obscured their differences about the war, the Democratic speakers have been unified and full-throated in their depiction of Americans' economic anxiety, as in Mr. Edwards's portrayal last night of "two different Americas." They argue that new technology, globalization and other forces of change have left workers less secure than reassuring statistics on unemployment and overall economic growth would suggest. The keynote speaker, Senate candidate Barack Obama, described the workers in his home state of Illinois "who are losing their union jobs at the Maytag plant that's moving to Mexico and now are having to compete with their own children for jobs that pay seven bucks an hour." Mr. Edwards invoked the image of a worried mother, sitting alone in her kitchen, unable to pay her bills. "She's doing everything right, but she still can't get ahead," he said.

Will Mr. Kerry convince voters that he offers a plausible strategy to relieve this "middle-class squeeze," as the Democrats are calling it? So far, Democrats have argued persuasively that Mr. Bush has failed to do so and has exacerbated one arguable consequence of globalization -- rising economic inequality -- with tax cuts skewed to the wealthiest Americans. But their own solution is still taking shape. At times the Democrats seem to argue that they can slow change by restricting trade or damming the flow of jobs overseas by tinkering with the tax code. "We will . . . bring good-paying jobs to the places that need them," Mr. Edwards vowed last night -- as if that magic trick could be conjured by a Kerry administration. At other times Democrats argue, more convincingly, that they can help relieve the anxiety by assuring workers that they will have health care and enough money to retire on, and that their children will have access to college, even if their jobs disappear. Mr. Kerry has fashioned a menu of specific proposals during his months on the trail. Tonight will be a test of whether he can knit them together into a vision for the future that, without pandering or overpromising, presents a credible alternative.