On the day last September when President Clinton summoned his Cabinet to the White House residence to apologize for lying about his affair with Monica S. Lewinsky, Vice President Gore sat by his side.

As the meeting gained emotional force -- Clinton at times was near tears, and several participants laced their comments with biblical references -- the vice president listened quietly, almost stoically.

When it was Gore's turn to speak, he sounded more like a father lecturing a wayward son than a vice president addressing his boss. In a tone that was "stern" but not harsh, according to two participants, Gore told Clinton he was "disappointed" in him. He told the group it was time to rally behind the beleaguered president.

Then he concluded with a remarkable admonishment: "Mr. President, I think most of America has forgiven you, but you've got to get your act together."

Gore's role that day -- as it has been many times over the past seven years -- was to steady Clinton's presidency at a time when it was listing badly. But his motives, say associates, were complex: Gore feared Clinton's tangled past was becoming a danger to his future.

This morning, Gore will announce his long-planned candidacy for president on the steps of the Smith County Courthouse in his home town of Carthage, Tenn.

Clinton will be far away, in Europe for a summit. But his presence promises to be hovering over everything Gore does during the opening phase of his campaign, as Gore seeks to reintroduce himself to Americans as more than a dutiful subordinate -- a leader who is a fully credible occupant of the Oval Office.

Two southern baby boomers whose mutual presidential ambitions first brought them into contact a dozen years ago are entering what one confidant of both describes as "the most difficult phase" of their relationship.

Clinton is being shunted somewhat unwillingly to the background; Gore is stepping into a spotlight that still seems at times to leave him uncomfortable.

For years, both Clinton and Gore have worked assiduously to create a public image of their partnership. Gore is the "most influential and effective vice president in the history of the United States," Clinton has said. Gore has been unflaggingly loyal to a man he describes as "my friend."

This public image, say many associates of both, is accurate -- but incomplete. It masks more complicated facets between two men who often view each other with a mix of mutual admiration and incomprehension.

It was this incomprehension -- and anger -- that colored Gore's views of the Lewinsky controversy. In the weeks after Clinton's public acknowledgment of the affair, Gore placed urgent calls to his most trusted advisers. "He was dismayed by what the president had done," explains one friend, "and more importantly, dismayed the president put so much at risk."

Clinton, for his part, remains baffled by what he sees as his understudy's frequent political clumsiness, according to sources.

"Clinton has put an awful lot of political capital in Al Gore," says one confidant of the president's. "What is frustrating to Clinton is that given all the advantages, all the latitude, all the exposure, all the things he has let Gore take the lead on, he can't seem to get it."

The president, this person says, believes Gore "would be a good president" but is uncertain he can win. "It's so frustrating because to Clinton it comes as second nature and with Gore, it's a constant strain."

For all the similarities in age and political outlook, they are vastly different men. "They are more ideological and philosophical buddies than locker room buddies," as Energy Secretary Bill Richardson put it.

Before they ever hopped a campaign bus together in 1992, the two men viewed each other as rivals. Looking "from afar across state lines," recalls Gore adviser Roy Neel, "they were virtual competitors as comers" in Democratic politics.

The two first met in 1988 when then-Sen. Gore traveled to Little Rock seeking Gov. Clinton's presidential endorsement. He didn't get it, and four years later when Clinton ran, Gore did not back him.

Yet by June 1992, after then-Joint Chiefs Chairman Gen. Colin L. Powell and then-Sen. Bill Bradley (D-N.J.) indicated they were not interested in the No. 2 spot, Gore quickly moved up the list, say several involved with the selection. His appeal was his clean record and a resume that balanced some of Clinton's vulnerabilities -- he had Vietnam service, a happy marriage and foreign policy expertise.

Neel says Gore returned from a late-night session with Clinton "surprisingly buoyant." The June meeting at the Capital Hilton lasted 3 1/2 hours and although Clinton held similar sessions with other candidates, Gore "felt it was an extraordinary meeting and that they hit it off," Neel recalls.

For the next month, a team of lawyers led by Warren Christopher and Harry McPherson "vetted" Gore and the other prospects. "The thing we found strange was how few friends Gore had," recalls one member of the team. "Particularly for someone like Clinton who had so many friends, picking a guy with such a different temperament and different social ability was of concern."

Clinton was briefed in memos and conversations on Gore's "disastrous" 1988 campaign, the source says. But friends were summoned to attest to his nicer side and several described how Gore had learned from the defeat, using the subsequent four years to write a book, burnish his fund-raising skills and repair relations with leaders such as Jesse L. Jackson.

Gore, after being selected, had his priorities. After a whirlwind convention in New York, the Clintons and Gores boarded a bus for what was to be a two-day trip together. But Gore balked, battling with campaign scheduler Susan Thomases and eventually making a direct appeal to Clinton. He wanted to stay on the bus longer, by Clinton's side, at the center of the action. He brought a self-protective instinct to the relationship that continues to this day.

"Al was knowledgeable enough of how these campaigns happened, to make sure they didn't send him off to political Siberia," says Neel. In fact, Gore had spent much of the spring studying the role of vice presidents and was determined not to repeat history.

After the election, Gore moved to a hotel suite in Little Rock and Neel negotiated the terms of the partnership. Gore secured a weekly lunch date with Clinton, a role in hiring decisions and protection of his own budget "so we wouldn't get nickeled and dimed" by the White House, Neel says.

Today, Gore loyalists are sprinkled throughout the federal bureaucracy and the Democratic National Committee and despite an early effort to eliminate his White House office, the vice president is a strong presence there, sometimes to a comical degree.

"He made an extraordinary effort to be in the loop, at meetings, always visible," recalls one official from the first term. "I had seen him on more than one occasion literally rushing down the hallway to a meeting in the Oval he had just learned about."

Yet Gore has always known the limits of his job. Explains his adviser Elaine Kamarck: "For the first term there was never a strategic difference between the two of them because so much of Al Gore's future depended on Bill Clinton having a good first term and being reelected."

He survived early tensions with Hillary Rodham Clinton when her health care overhaul and his "reinventing government" plan competed for the president's attention. And on major issues such as military intervention in Bosnia, NAFTA, environmental protections and the 1996 welfare reform, Gore is credited with forcefully shaping the outcome. He also is the point man on a host of second-tier issues that are nonetheless close to his interests and useful to defining an appealing political image. These include promoting NASA and the Internet as well as finding ways to curb television violence.

Gore has had failures over the years as well: He fought unsuccessfully for an energy tax and high-speed rail subsidies. Some Clinton advisers say Gore has been less effective than they thought someone who had spent 16 years in Congress would be at congressional relations.

Former Clinton political consultant Dick Morris, who worked closely with Gore when he was battling for position against other Clinton advisers in 1995, said the vice president is "co-president on about one-third" of the administration's agenda.

Officials say the sharp-witted Gore has an ability to defuse Clinton's legendary tantrums, often with the deadpan humor he saves for private settings. White House aides breathe easier when Gore attends the practice sessions for Clinton news conferences.

"If Clinton responded to a practice question with a self-pitying rant, Gore would clap his hands and spike his fist across the Cabinet table," says George Stephanopoulos. "Great answer, Mr. President. You got that one down!" Gore would exclaim. "Clinton got the message."

Though Gore and Stephanopoulos often clashed in the early days of Clinton's term, the former White House aide says the vice president's strategy for making himself indispensable to Clinton worked: "He went from fighting for position to having it."

Al From, head of the centrist Democratic Leadership Council, says, "Gore has brought unrelenting support. For somebody like Clinton who ebbs and flows, Gore gives a sense of stability."

The question is whether Gore's current position is an asset as he seeks to move to a new one. It promises to be one of the key puzzles of 2000 politics: Whether the popular but tarnished president helps or hinders his chosen heir's efforts to succeed him.

Unlike some past presidents, there is no doubt that Clinton is committed to electing Gore. A Gore victory would amount to a validation of the Clinton record. There is little doubt that Clinton has already helped Gore immeasurably: The access, visibility and platform that Clinton has accorded Gore as vice president is plainly one of his most valuable assets.

But Gore also carries burdens -- sometimes for being too close to Clinton, at other times for being less than his peer politically. Gore's team privately suggests there is one virtue to being measured against Clinton. Put simply, Gore may have problems, but he will never have a Monica problem.

"They are different people with different styles who conduct themselves differently in both the public and private arenas," says Richardson.

Yet Gore begins his 2000 marathon carrying Clinton baggage. Whatever private misgivings he may have had about the president's personal conduct, he soldiered loyally in public. Most famously, on Dec. 19, the day Clinton was impeached, Gore appeared at a South Lawn pep rally to say the vote "does a great disservice to a man I believe will be regarded in the history books as one of our greatest presidents."

Now, however, Gore is blunt in his criticism of the president's affair: "I want you to understand that there shouldn't be any mystery," he told ABC's Diane Sawyer in an interview to air on "20/20" tonight. "I thought it was awful, I thought it was inexcusable. But I made a commitment to serve this country as vice president."

Unlike Vice President George Bush, who waited until his "kinder, gentler" convention speech to draw a demarcation line with President Reagan, Gore is beginning the break now.

"There will be more and more of a coming-out" from Clinton's shadow, says Kamarck. "The vice presidency is not a good place from which to define yourself," she added, referring to polls showing the public knows little of Gore beyond his statue-like performances as loyal No. 2.

It will not be an easy transition for either man. Some Gore advisers say they are hoping Clinton can resist the urge to be Gore's de facto campaign manager and instead give him the space he needs to sharpen his persona.

Even today,as Gore opens the next phase of his political career, he remains in many respects right where he started, tied to Bill Clinton.

"Gore is in a difficult position," says From. "He has to do a message of continuity and change."

Staff writer John F. Harris contributed to this report.

CAPTION: Vice President Gore greets businessman Bill Markham in his home town of Carthage, Tenn. At left is Tony Coelho, general chairman of Gore campaign.