WASHINGTON — Summoning memories of his brother, the slain president, Sen. Edward M. Kennedy led two generations of the First Family of Democratic politics Monday in endorsing Barack Obama for the White House, declaring, ‘‘I feel change is in the air.’’

Obama is a man of rare ‘‘grit and grace,’’ Kennedy said in remarks salted with scarcely veiled criticism of the Illinois senator’s chief rival for the presidential nomination, Sen. Hillary Rodham Clinton, as well as her husband, the former president.

Obama beamed as first Rhode Island Rep. Patrick Kennedy, then Caroline Kennedy and finally the country’s best known liberal took turns bestowing their praise. ‘‘Today isn’t just about politics for me. It’s personal,’’ Obama told a boisterous crowd packed into the American University basketball arena a few miles across town from the White House.

It was also about politics, though, and a rapidly approaching set of primaries and caucuses across more than 20 states on Feb. 5, with more than 1,600 national convention delegates at stake.

Kennedy’s endorsement was ardently sought by all three of the remaining Democratic presidential contenders, and he delivered it at a pivotal time in the race. A liberal lion in his fifth decade in the Senate, the Massachusetts senator is in a position to help Obama court voting groups who so far have tilted Clinton’s way. These include Hispanics, rank-and-file union workers and lower-income, older voters.

Kennedy is expected to campaign actively for Obama beginning later this week, beginning in Arizona, New Mexico and California. Caroline Kennedy, the daughter of John Kennedy, who was assassinated in 1963, will also make campaign appearances, officials said.

David Axelrod, a senior Obama adviser, said strategists also hope Kennedy can help blunt Clinton’s charges that Obama’s health plan would not provide coverage for all. ‘‘I don’t think anybody believes that Ted Kennedy would endorse a candidate who wasn’t thoroughly committed to the goal of universal health care,’’ he said.

Clinton betrayed no disappointment at her rival’s gain.

‘‘We’re all proud of the people we have endorsing us,’’ she said in a conference call with Arizona reporters. Addressing Kennedy’s criticism of politicians who pit groups against one another, she said she was ‘‘strongly in favor of getting to where our politics can be about the real issues, trying to find common ground.’’

So strong is the Kennedy family’s hold on some Democrats that as word spread on Sunday about the elder Kennedy’s plans, Clinton announced that she had the backing of Kathleen Kennedy Townsend. A daughter of Robert F. Kennedy, Townsend lost the gubernatorial election in Maryland five years ago.

In his remarks, Kennedy methodically sought to rebut many of the arguments leveled by Obama’s critics.

‘‘I know he’s ready to be president on day one,’’ Kennedy said, taking on one of Bill Clinton’s frequent talking points.

‘‘From the beginning, he opposed the war in Iraq. And let no one deny that truth,’’ he said, an apparent reference to the former president’s statement that Obama’s early anti-war stance was a ‘‘fairy tale.’’

‘‘With Barack Obama, we will turn the page on the old politics of misrepresentation and distortion.

‘‘With Barack Obama we will close the book on the old politics of race against race, gender against gender, ethnic group against ethnic group, and straight against gay,’’ Kennedy said.

The Massachusetts senator had remained on the sideline of the presidential campaign for months, saying he was friends with Obama, Clinton and former North Carolina Sen. John Edwards, as well as several Senate colleagues who are no longer in the race.

Lately, according to several associates, Kennedy became angered with what he viewed as racially divisive comments by Bill Clinton. Nearly two weeks ago, he played a personal role in arranging a brief truce between the Clintons and Obama on the issue.

The day’s political drama continued into the evening. Kennedy followed Obama into the House chamber Monday night for President Bush’s State of the Union address. As Hillary Clinton and Kennedy shook hands before the speech began, Obama turned away from her.

Obama, 46, is nearly 30 years younger than Kennedy. ‘‘I was too young to remember John Kennedy, and I was just a child when Robert Kennedy ran for president,’’ he said. ‘‘But in the stories I heard growing up, I saw how my grandparents and mother spoke about them, and about that period in our nation’s life — as a time of great hope and achievement.’’

Kennedy usually refers only sparingly to his assassinated brothers, John and Robert, in his public remarks, and his endorsement of Obama was cast in terms that aides said were unusually personal.

‘‘There was another time, when another young candidate was running for president and challenging America to cross a new frontier. He faced criticism from the preceding Democratic president, who was widely respected in the party,’’ Kennedy said, 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.’

‘‘So it is with Barack Obama,’’ he added.

Kennedy began by paying tribute to Sen. Clinton’s advocacy for issues such as health care and women’s rights. ‘‘Whoever is our nominee will have my enthusiastic support,’’ he said.

But he quickly pivoted to a strong endorsement of Obama, who he said ‘‘has extraordinary gifts of leadership and character, matched to the extraordinary demands of this moment in history.’’

‘‘I believe that a wave of change is moving across America,’’ Kennedy said.

Rep. Patrick Kennedy, the senator’s son, completed the family tableau onstage with Obama. The congressman said, ‘‘In times such as these, we need, as we had with my uncle, a leader who can inspire confidence and faith in our government. A sense that our government can be good again.’’