Other than his victory in the Iowa Democratic caucuses, no moment catalyzed Barack Obama's historic presidential campaign more than winning the endorsement of Sen. Edward M. Kennedy.

At a Jan. 28, 2008 rally at American University in Washington, the then 75-year-old progressive warrior from Massachusetts forcefully rejected arguments that Obama was inexperienced and not ready to lead the nation, and drew parallels to the path breaking campaign his brother waged in 1960.

The address at once firmed up Obama's bona fides with unions, Latinos and senior citizens and dealt a staggering blow to the presidential hopes of his chief primary rival, Hillary Rodham Clinton.

In return for his support, Kennedy made Obama pledge to make health care a first-tier priority -- a promise the president fulfilled by staking much of his first-term agenda on an ambitious and controversial plan to retool the U.S. health care system.

Veteran lawmakers -- including Obama's campaign opponent, Arizona Republican Sen. John McCain -- have lamented Kennedy's absence from the Senate during this year's health care debate and speculated how his presence might have by now helped forged consensus on the contours of a plan.

Obama acknowledged his indebtedness to Kennedy on Wednesday while vacationing on Martha's Vineyard, calling him "not only one of the greatest senators of our time, but one of the most accomplished Americans ever to serve our democracy."

"The Kennedy name is synonymous with the Democratic Party. And at times, Ted was the target of partisan campaign attacks," Obama said. "But in the United States Senate, I can think of no one who engendered greater respect or affection from members of both sides of the aisle. His seriousness of purpose was perpetually matched by humility, warmth, and good cheer. He could passionately battle others and do so peerlessly on the Senate floor for the causes that he held dear, and yet still maintain warm friendships across party lines."

In a written statement the White House released early Wednesday morning, the president recalled how Kennedy found time to make him feel welcome in the Senate, in spite of the swirl of legislative activity, and provided advice and counsel.

"I cherished his confidence and momentous support in my race for the presidency," Obama said. "And even as he waged a valiant struggle with a mortal illness, I've profited as president from his encouragement and wisdom."

Kennedy's endorsement provided Obama's campaign enormous credibility with the political left and also invoked poignant memories of his slain brother.

"There was another time, when another young candidate was running for president and challenging America to cross a new frontier," Kennedy said at the American University rally.

"He faced criticism from the preceding Democratic president, who was widely respected in the party," Kennedy noted, referring to Harry S Truman. "And John Kennedy replied: 'The world is changing. The old ways will not do. It is time for a new generation of leadership.' "

Kennedy also provided one of the most moving moments in the campaign, when he appeared on the first night of the Democratic convention in Denver, less than three months after undergoing surgery to remove a brain tumor, and delivered a fiery speech to delegates holding blue placards resembling his 1980 campaign signs.

Some of Kennedy's Senate colleagues on Wednesday said his negotiating savvy and ability to reach across the aisle would likely have Congress and the administration closer to an agreement on a health care plan.

"Had his own health allowed him to fully participate, we would be far closer to consensus today on a path to health care in America," said Delaware Democratic Sen. Thomas R. Carper.

Kennedy's death could change the tenor of the debate, now mired in fierce partisan battles over how to pay for an overhaul and what role the government should play in a retooled health insurance market.

Observers believe fiscally conservative and centrist Democrats, particularly in the House, might be more inclined to take a tough vote on a health care bill if it is linked to the legacy of the late senator.

But Kennedy's death could also underscore the lack of a true legislative leader in the health debate. The power vacuum, if it remains unfilled, might make undecided House members and senators more reluctant to embrace any compromise that smacks of liberal activism or expensive social engineering.

Anna Burger, chairwoman of the labor coalition Change to Win, on Wednesday linked the health care overhaul to Kennedy's work on social justice issues, saying, "The most fitting tribute to honoring the life and legacy of this great statesman is for Congress to pass quality affordable health care for all this year."

Jack Lewin, chief executive officer of the American College of Cardiology, predicted Kennedy's death increases prospects for a bipartisan compromise, even at a time when the financial world is jittery about the prospect of more federal spending adding to the national debt.

"The sentiments and loyalties to the Lion will certainly nonetheless tilt the Congress' agenda toward passage of some kind of historic health reform bill. Whatever it contains, it will be more than it might have been in his honor and memory," Lewin said.