As former aides, historians and journalists met recently at a celebration of Lyndon Johnson's 100th birthday, a persistent theme was the way echoes of his presidency still reverberate through the nation's politics.

Four decades after Johnson's inability to settle the Vietnam War destroyed his presidency, the nation is coping with how to extricate itself from another quagmire - in Iraq.

But the impact of his landmark domestic achievements, especially the historic measures empowering America's minorities, also is evident in the prospect that the nation may elect its first black president.

Indeed, noted onetime LBJ counsel Harry McPherson, Johnson would see Barack Obama's candidacy as a direct legacy of his efforts to expand voting rights of blacks and other minorities.

And the massive Democratic primary turnouts, especially among younger voters, may suggest the country "really not only wants new leadership but wants a new generation of leadership" as in the 1960s, said historian and former journalist Nick Kotz.

Already, it appears likely the Democrats will significantly increase the House and Senate majorities they won two years ago. And the anti-GOP climate means they have a good chance of regaining the presidency, giving them control of both government branches for the first time in 14 years and the ability to tackle a long-stalled national agenda, headed by health care.

"You have the potential for a brand new progressive wave that we haven't seen since 1964 with Lyndon Johnson," historian Douglas Brinkley said.

But it's a long time until November. And the lesson of LBJ's presidency is not only the extent to which an inspirational president can capitalize on unique political circumstances to achieve major changes, but is also the rarity of such moments in U.S. history.

Johnson's Great Society marked the first such period since FDR's New Deal lost its legislative clout a quarter-century earlier. The Texan's initiatives enjoyed clear public and congressional support for less than three years, until Democrats lost the seats in 1966 they had won two years earlier.

Since then, only Ronald Reagan in his first year had the clout to enact a sweeping program. Ironically, his sweeping tax and budget cuts signaled a partial rollback of LBJ's Great Society.

Even if Democrats gain the majorities this November to surmount hard-core Republican resistance, the trauma of withdrawing from Iraq and the dire fiscal situation they will inherit will limit their ability to enact sweeping legislation.

"The tragedy of Vietnam," Joseph Califano said at the Johnson celebration, "has created a dark cloud obscuring the full picture of Lyndon Johnson's presidency," so even natural Democratic heirs seldom cite him.

In making poverty the centerpiece of his presidential campaign, John Edwards "never mentioned Lyndon Johnson, the first - and only - president ever to declare war on poverty and sharply reduce it," said Califano, a former Johnson aide who served later as secretary of health, education and welfare.

When Obama hails the achievements of Presidents Roosevelt, Truman and Kennedy, he omits "the man who would be proudest of Barack Obama's candidacy and what it says about America," Califano noted.

In the end, events may compel Obama to cast his own historic presidential bid in the context of the two men whose efforts did so much to make it possible - LBJ and the Rev. Dr. Martin Luther King Jr.

Democrats will formally nominate their standard-bearer Aug. 27, Johnson's actual 100th birthday. And the nominee's acceptance speech is set for the next night, the 45th anniversary of Dr. King's famous "I Have a Dream" speech at the 1963 civil rights march on Washington.

Carl P. Leubsdorf (cleubsdorf@dallasnews.com) is Washington bureau chief of the Dallas Morning News.