As Sen. John Edwards campaigns nationwide for John F. Kerry -- and for the second spot on his presidential ticket -- the North Carolinian's main strength is also his biggest handicap, a paradox at center stage of Kerry's narrowing search for a running mate.

Of all the surrogate candidates and vice presidential hopefuls, none can touch Edwards's ability to electrify crowds and charm voters out of their socks. But Democratic big shots and small-county chairmen alike say there is no question that the charismatic senator still covets the presidency -- in 2008, 2012 or whenever the next opportunity arises.

Among the biggest decisions Kerry faces is whether Edwards could check his ego and ambitions for four or eight years and play the loyal, subservient and rarely glamorous role of vice president, whose greatest concern is supposed to be the president's best interests. Purely in terms of campaigning this fall, the Massachusetts senator also must consider whether Edwards's sizzle would make his own more prosaic style seem unacceptably wooden by comparison.

This weekend in this mecca of presidential politics, Edwards's star qualities were in full glory, delighting hundreds of Iowa Democratic convention-goers but surely prompting mixed feelings inside the Kerry camp. In speeches Friday night and Saturday morning, Edwards ripped the Bush administration and repeatedly brought the delegates to their feet, whooping, cheering and later mobbing him for photos and autographs as the party convention tried to resume its business.

Edwards heaped praise on Kerry, calling him "a man of strength, courage, determination, leadership, vision." He refused to answer reporters' questions about the vice presidency, saying his visits here and to dozens of other cities are made solely to elect Kerry. But in his first visit to Iowa since the January caucuses -- in which Kerry finished first, Edwards ran a strong second and former Vermont governor Howard Dean imploded -- Edwards reprised lines from his old stump speech, high-decibel riffs that might echo in Iowa caucus participants' heads three years from now.

"That's the America you and I will build together!" he shouted at the end of Friday's speech, after painting a vision of equality and opportunity. The roaring crowd nearly drowned out his adding, "That's the America we are fighting for to elect John Kerry."

The room held another politician supposedly on Kerry's short list for a running mate, but an outsider would hardly have guessed it. Iowa Gov. Tom Vilsack addressed the audience Friday before Edwards spoke, but he kept his voice calm and his comments largely generic, such as, "Is it great to be a Democrat or what?"

Vilsack and Edwards exchanged lavish compliments in their speeches, but the governor made no effort to challenge the visitor's marquee billing. While Edwards held a crowded news conference, Vilsack entered the convention hall quietly. When Edwards worked the room afterward, almost having to fight his way out the door 15 minutes later, Vilsack and his wife sat at a table. (It did not help the governor's cause that a story in Friday's Des Moines Register was headlined, "Iowans have hard time seeing Vilsack as candidate.") A key question among Democratic insiders is: Do Edwards's attention-grabbing skills and bubbling ambition -- especially in light of his mere five years in public office -- rub Kerry wrong? Plenty of rumors and speculation among Democrats suggest they do. But colleagues and consultants who know the two men well say the stories are often exaggerated and their relationship cannot be easily labeled.

"They had a cordial working relationship in the Senate," said a Democratic consultant familiar with the situation who spoke on background because of the topic's sensitivity among party leaders and Kerry. "They weren't particularly close, but there wasn't friction."

Numerous Senate and campaign insiders agree with that view. Some say Kerry felt an early and understandable jealousy in 2000 when presidential nominee Al Gore seemed to give the two men roughly equal consideration as his possible running mate, even though Kerry was a three-term senator and former lieutenant governor and Edwards had never run for office until two years earlier.

"It was sort of disbelief" that Edwards's star had risen so quickly, said another Democratic consultant who knows Kerry well. But the greater threat to a friendship between the two, this consultant said, came in 2003, when Edwards made a full-bore run for president alongside Kerry, Rep. Richard A. Gephardt (Mo.), Sen. Bob Graham (Fla.) and others who had tried before or nursed their ambitions for years. "Kerry did think Edwards was shockingly presumptuous for doing this," the consultant said. "It got under his skin to see Edwards being the flavor of the month in 2003."

Perhaps the senator who knows them best, Edward M. Kennedy (D-Mass.), says Kerry got beyond those feelings, coming to admire Edwards's political skills even as the two competed for the nomination. "I think they've got a good relationship," Kennedy said in a recent interview. "I think in the course of the primaries it was tense. But in talking to John [Kerry] after the primaries were over, there was a kind of a bond that formed between these people."

Kennedy, considered a mentor to Edwards and a strong champion of Kerry's bid, said he has made no effort to sway Kerry's choice. "I told John whoever he chooses, and whoever he thinks will make the biggest difference for him in terms of meeting the responsibilities and having someone who can succeed if something should happen to him, and having someone he can work with" will be fine. "I don't see any tea leaves suggesting his choice."

Usually circumspect, Kerry occasionally has questioned Edwards's experience in public affairs, especially foreign and military matters. He once said that when he returned from Vietnam in 1969, Edwards (born in 1953) might not have been "out of diapers." He later called to apologize.

The men's sharpest exchange came in New York on Feb. 29, in the last presidential debate before Kerry's Super Tuesday triumph, which ended Edwards's bid. Edwards, needing to distinguish himself from the front-runner, portrayed Kerry as a Washington insider who backed trade pacts that killed U.S. jobs. "This is the same old Washington talk that people have been listening to for decades," he said of Kerry's remarks on taxes and spending.

An irritated Kerry shot back that Edwards had a newfound interest in protectionism. As for Washington insider status, Kerry said acidly, the "last time I looked," Edwards was a U.S. senator.

Since then, Edwards has joined Gephardt, Vilsack and others who are stumping for Kerry and, without ever acknowledging it, auditioning for the ticket's second spot. The biggest concern for Edwards's supporters, who include many lawmakers and state party leaders, is that Kerry will opt for a deeply experienced colleague whose presidential fires have cooled.

"He's going to want someone he can trust a million percent," said the consultant who knows Kerry well. "I think Gephardt fits that criteria," whereas "Edwards's ambitions are so overt."

Edwards fans hope there was no omen in a Los Angeles debate three days before the New York encounter. Asked to contemplate running together, Edwards said cheerily, "An Edwards-Kerry ticket would be powerful."

Kerry thanked him "for the consideration." But he made no mention of a Kerry-Edwards ticket.

Sen. John Edwards, with Democrats in Iowa, called Sen. John Kerry "a man of strength, courage, determination."