John F. Kerry, who has struggled to find a clear and imaginative way to fuse a biography of public service to a vision for the nation, Thursday night will strike a highly personal tone, with nods to history and family, in the highest-stakes speech of his political life, according to aides.

The challenge is clear. Still an amorphous political figure to most Americans, Kerry must demonstrate he is strong enough to fight terrorism and appealing enough to win over undecided voters. And the candidate known for long, meandering and sometimes arcane orations must do so with the clarity of Bill Clinton on Monday and the conviction of Barack Obama on Tuesday, Democrats say.

"His task is really to clarify and simplify and boil down his vision," said Harold Ickes, former White House deputy chief of staff in the Clinton administration. "He hasn't come close enough yet. . . . But these are works in progress. I'm absolutely confident Kerry will develop a sharper focus."

After 18 months of full-time campaigning, polls show that Kerry remains something of an enigma to many voters -- a candidate who has run through a series of campaign slogans and grappled with the right mix of ideas to capture how he would lead as president. Last week alone, Kerry focused alternately on values, national security, optimism, technology and science, but there was rarely a unifying theme.

Many Democrats say when Kerry steps onto the podium in FleetCenter, he will be in an enviable and difficult position -- enviable because about half the country appears willing to turn Bush out of office, difficult because Kerry alone must make the sale and will have few better opportunities to make his case between now and November.

Kerry, who professes a love of poetry, history and the written word, penned the speech longhand with the input of a few speechwriters, family members and letters his father wrote his mother long ago, aides said. He spent at least 30 minutes a day for the past month, including a recent weekend in Nantucket, refining it.

Early on, Kerry sought input from several speechwriters, many outside his campaign. But a top aide said Kerry was persuaded to work from fewer than a half-dozen drafts written, for the most part, by Robert Shrum, a longtime adviser and one of the party's best-known wordsmiths, and Terry Edmonds, a former Clinton speechwriter. Kerry, often writing in the morning or aboard his campaign plane, would literally cut out sections he liked and paste them into his working draft, a top aide said.

Kerry seemed focused on sharing more of himself in this speech, the aide said. He talked with brother Cameron about childhood memories, and read from journals he has kept over the years. Kerry would read parts of the speech to staff, but most of his practice time was before his wife or daughters.

The day before Kerry arrived here in Boston, one senior adviser said the speech was mostly done, except for the inevitable final tweaking, and described it as quite personal in tone. "It's not a heavy, policy-laden speech," the adviser said, asking not to be identified so as to talk more openly about the process of preparing the speech. "It's not a laundry list" such as the one former vice president Al Gore did in 2000.

Unlike the last two men to win the White House, Kerry prepares to accept his party's presidential nomination without having found the shorthand phrase -- such as Clinton's "New Democrat" or Bush's "compassionate conservatism" -- to capture what his candidacy is about.

But some veterans of the 1992 campaign said Kerry's challenge is far easier than what Clinton faced in his acceptance speech. "We forget this now, but he went into the convention with most people only knowing about Gennifer Flowers or draft dodging, and most people before the convention assumed he was a rich kid whose daddy bought him out of the draft," Democratic strategist Mandy Grunwald said.

"We had to undo things," she added. "With Kerry, he needs to deepen impressions and give people a fuller sense of who he is. There's more a lack of information, not misinformation."

Grunwald likened Kerry's challenge to what faced Ronald Reagan in 1980, which was to reassure voters ready to change presidents that they could entrust the White House to him. "This country wants change, but they're not sure about John Kerry," she said. But, one Democrat noted, unlike Reagan, it is not clear Kerry has a black-and-white set of beliefs he can communicate to voters even if they are willing to give him a serious look.

Andrei Cherney, former director of speechwriting for the Kerry campaign, agreed that Kerry is in a position familiar to many challengers. "People know a little about him -- some details of his biography," he said. "This is the time where he is going to have to both tell his story, tell the country his vision and connect the two."

A recent Washington Post-ABC News poll found a significant number of Americans who said they want to know much more about Kerry's positions on major issues, but strategists who have worked in previous campaigns say the goal of an acceptance speech should not be the enumeration of 5-point plans on health care or the economy. Instead, they said, what is crucial is for the nominee to make a connection and leave an impression on the television audience. "What matters is that they're comfortable," Grunwald said. "They want to get a gut sense of who he is and whether he can do the job and whether he can do it in a way that benefits them."

The tightly controlled themes of this convention reflect two of Kerry's main goals with the speech: appear strong on national security and optimistic in tone. A Kerry adviser said that while Kerry plans to talk about his agenda for national security, the economy, health care, energy independence and other issues, he does not plan to dwell on the details. The goal, this adviser said, is to convey a persona that voters can identify with. "It could be character, it could be personality, it could be leadership, it could be humor," the adviser said. "It's putting texture to the person."

Where some Kerry advisers part company with outside strategists is in their belief that the Massachusetts senator already has passed a crucial threshold in the campaign. Voters, they said, see him as capable of being commander in chief. "The American people know he can lead the country," the adviser said. Democrats outside the campaign are far less sure, saying that with so many Americans still not certain they know enough about Kerry, demonstrating his fitness to be commander in chief is still the most important test he must pass. Given all of the attention to national security in this campaign, and the effort to surround Kerry with former crewmates from Vietnam and retired military brass, campaign officials at least implicitly agree.

Cherney described Kerry as a politician who believes in the power of words and speeches to move an audience and move his campaign, and he cites several of Kerry's past speeches as memorable -- from his early assertion that Democrats should challenge Bush on national security when most Democrats were afraid to do so to his Jefferson-Jackson Day speech in Iowa last November when he began to turn his candidacy around.

But the most memorable may be his formal announcement speech in South Carolina last September, when competing camps within the campaign fought over their respective drafts. The final version was so badly received that Kerry ordered it rewritten for the second stop of his announcement tour.

Cherney said the South Carolina experience proved that Kerry controls the final product. "What you're going to hear him say is something he's 100 percent comfortable with and not something derived from outside pressures or sources," he said. "At the end of the day, he's going to give his speech."

Kerry advisers said the candidate hopes to use his speech to describe the values and experiences that shaped his life and connect them to an aggressive forward-looking agenda for the presidency.

"A lot of it is showing that he's done it before," one adviser said. "He's fought the tough fights. He's fought on the right side of people. This week is about surrounding him with people who've been in those fights with him and trust him as a leader, trust him as a friend and trust him to do the right thing. It's not just about that speech; it's about this week and shaping that premise that he fights on the right side of America."

John F. Kerry waits for his wife's speech as he watches convention coverage Tuesday. He prepared for the nomination by drafting his speech longhand.