John F. Kerry has been phoning friends at all hours and reviewing vice presidential choices dating to 1932 as the Democratic candidate nears what many consider the biggest -- and most telling -- decision of his general-election campaign, according to Democrats inside and outside the campaign.

While Kerry is tight-lipped about the pick, these Democrats said the process is revealing much about how the senator from Massachusetts views his strengths, his leadership style and the role of a vice president. Kerry has privately expressed confidence that voters see him as sufficiently strong on national security, they say, but wonders whether he needs a moderate or conservative Democrat on the ticket to improve his centrist credentials. Still, Kerry is skeptical a running mate can make a decisive difference in the election's outcome, these sources said, and is much more concerned with finding a ready-made president, though one who will not try to steal the show.

Sen. John Edwards (N.C.) has emerged as the favorite of many Democratic senators and Kerry friends and advisers. Edwards's stock has shot up in recent weeks as private polling shows the freshman senator providing a boost to the ticket in key states because of his southern appeal and perceived likeability, two sources close to the campaign said. "The delay in announcing someone has helped Edwards," a Democrat close to Kerry said.

Kerry, who had been enamored with the idea of a unity ticket, all but dropped that as a possibility after Sen. John McCain (R-Ariz.) rebuffed his overtures, the sources said.

Many of his friends said Kerry showed where they think his heart is by giving a 90-minute interview Wednesday evening to another rival from the primaries, Rep. Richard A. Gephardt (Mo.). Kerry considers the former House minority leader not only presidential and trustworthy but also unlikely to upstage or overshadow him on the campaign trail or in office, the sources said. Iowa Gov. Tom Vilsack and Sen. Bob Graham (Fla.) appear very much in the running, too. Kerry met privately with Graham two weeks ago in Florida.

Two Democrats close to Kerry said retired Gen. Wesley K. Clark's stock plummeted after the former Democratic presidential candidate received lackluster reviews from some former colleagues.

Women and minorities are rarely cited by Kerry's allies as top contenders. But history shows the eventual vice presidential pick is often someone the campaign and media have not mentioned. Think Richard B. Cheney in 2000.

The Kerry campaign is preparing for a rollout ceremony announcing the vice presidential choice early next month that will hit a peak at the Democratic National Convention in Boston, which begins July 26. The idea is to generate as much excitement and interest as possible in the Democratic ticket before the Olympic Games and GOP convention seize control of August.

But on Thursday in Detroit, Kerry remained as tight-lipped as before about the process. "I'm the only person who knows when I will [make an announcement], or what . . . direction that might take. I intend to keep it that way," he told reporters.

Friends say Kerry believes he has passed a national security threshold with voters that has freed him to tap a vice presidential candidate who complements him in other ways. In 2000, Cheney was seen as a wise choice for Texas Gov. George W. Bush because he brought foreign policy credentials to the ticket.

Some Democrats said Kerry is less confident that he has successfully sold himself as a moderate politician. The campaign considers the South to be Kerry's biggest regional liability, which may bode well for Edwards. Some close to Kerry talk of a "Mississippi strategy" that would move away from a traditional battle for the South and focus on the states stretching from Louisiana, through Arkansas and Missouri, northward. This helps explain Kerry's early interest in Clark, who is from Arkansas, and midwesterners Gephardt and Vilsack, according a Democrat close to the campaign.

Some Democrats are pushing Sen. Evan Bayh (D-Ind.), who voted for some of President Bush's tax cuts, sending a clear signal of centrism. Bayh would "underscore" that Kerry "is bringing mainstream values to the ticket," said Democratic pollster Mark J. Penn, who is promoting Bayh.

Kerry told American Urban Radio Networks on Thursday that a prerequisite is "somebody who has the ability to fill in as president if something terrible were to happen." Most presidential candidates say this, but Kerry seems unusually sensitive to that possibility, according to several people who have talked to him. Kerry is a student of history and an ardent fan of President John F. Kennedy, who was assassinated. Kerry also saw death up close in the Vietnam War, and he faced prostate cancer in 2003.

Kerry has privately expressed concerns about whether Edwards meets this presidential threshold, the sources said. After the primaries, Kerry remarked to aides, "What makes him think he can be president?" Around that time, Kerry told aides that if he had lost the nomination, he would have endorsed Gephardt, who he described as ready-made for the job.

Unlike many Democrats, Kerry does not believe his vice presidential pick will make a huge difference in whether he wins the Nov. 2 election -- unless McCain had signed on, a top aide said. History has shown running mates rarely make or break a ticket but can provide an image boost, as Sen. Joseph I. Lieberman (D-Conn.) did for Al Gore four years ago.

Kerry has told friends it is important to select someone with whom he is personally comfortable. "A vice president doesn't necessarily help the ticket, so go with someone you're comfortable with," said a top adviser. Kerry and Edwards, who were the final two serious contenders standing in the Democratic primary season, had a cool relationship early on, but aides said it has warmed of late. "They are far enough away from the distraction of the primary process now and Kerry understands how much support Edwards has," a Democrat close to Kerry said.

But Kerry is closer personally to Gephardt, Vilsack and Sen. Joseph R. Biden Jr. (Del.), a dark-horse candidate.

Some aides expressed concern about selecting a running mate who harbors obvious ambitious for the White House in 2012, another aide said. A Kerry adviser said Vice President Gore's ambitions complicated the final years of the Clinton presidency because election-year politics was always a chief concern. But this was not in the marching orders Kerry gave his selection team, they said.

Kerry's competitive streak, which has run deep throughout his career, is also coloring his decision, friends say. Kerry, they say, sometimes appears conflicted when talking about his desire to find a strong leader, or a peer, who could without a doubt run the nation in wartime and his concern of being upstaged or unfavorably compared with his running mate, stylistically or professionally.

Cheney is figuring in Kerry's thinking. Kerry has told friends he wants a strong, loyal Democrat but not a co-president. He said that Cheney appears to have so much authority he often upstages the president, which, in turn, Kerry believes weakens the presidency.

"Cheney is the prime minister -- John would never allow that," said Richard C. Holbrooke, a Kerry friend and campaign adviser. "John has foreign policy experience. He does not need a prime minister."

Kerry is taking a slow, deliberative and very hands-on approach to selecting a vice president. Aides say it is a microcosm of the candidate's cautious decision-making process -- sometimes overly cautious, according to some Democrats. Only James Johnson, who is heading the campaign's search, and campaign manager Mary Beth Cahill are privy to the secretive process.

Kerry told Johnson he did not want to repeat the mistake Walter F. Mondale made in 1984, when the Democratic nominee was seen as interviewing people only to please constituencies, a Democrat close to the campaign said. Kerry also insisted to his campaign staff that he wanted to maintain a discreet process to avoid the spectacle of 2000, when Kerry was kept dangling for weeks by Gore before being passed over for Lieberman. After studying the process stretching to 1932, Kerry called for a long, secretive approach to the selection that would not leave him "jammed" at the end, making a hurried choice at the convention, as Kennedy did in 1960.

At Kerry's direction, Johnson has consulted broadly, touched all bases inside the Democratic Party and reached out to Republicans, according to numerous people contacted by Johnson. Kerry wanted detailed reports on the major public policy positions of the potential contenders, especially their views of Bush's agenda. Armed with this information, which he has analyzed and reanalyzed, sources say, Kerry has applied a prosecutorial tone by calling advisers and friends at all hours to discuss the pros and cons of certain candidates.

Many Democrats close to former president Bill Clinton, working at the Democratic National Committee and in Congress, are pushing hard for Edwards. If any Democrat is tirelessly auditioning for the job, it is Edwards, who is giving speeches, raising money and heading campaign rallies.

Edwards is a "southern centrist" and a "charismatic candidate," said Sen. Byron L. Dorgan (N.D.), who has discussed the pick with Kerry and Johnson.

Gephardt is widely respected by most Democrats but is seen as a less exciting choice. He is a skilled debater and a disciplined campaigner who never strays from message. He enjoys fervent backing from labor union leaders, some of whom privately express frustration with Kerry. Having run for president twice, and having been in a House leadership position, Gephardt has already been vetted by Republicans and the media. "There are no surprises with Dick. Everything is already in the record -- good and bad," one Gephardt ally said.

But Gephardt's assets could be his liabilities. During both his presidential races, he failed to demonstrate a broad national appeal, and he is viewed by many in his party as an old-school Washington politician.

Romano reported from Detroit.

In Detroit, John F. Kerry said, "I'm the only person who knows" when he will decide on a vice presidential choice.