Kennedy said "my endorsement is about the future. And it's Barack Obama. And it's about whether he is going to be able to win the nomination and get about the business of bringing both the party and the country together. And I believe he has."
"What today was about, at least, in my view … to the people of my state, who will be voting next Tuesday, about who I believe can help lead this country and lead the world."
Like her uncle, Caroline Kennedy stressed the importance of moving forward.
"This kind of person doesn't come along very often, and when they do, I think that it really is up to all of us to give it a serious look and put aside whatever plans we might have had and really get behind that kind of inspiration and power," she said.
One of the Democratic establishment's towering figures, Kennedy also shared the stage Monday with son Rep. Patrick Kennedy, D-R.I.
While the Clinton campaign would have loved the endorsement of the Massachusetts heavyweight, it's quick to point out that it has its own backing from other members of the Kennedy clan: Kathleen Kennedy Townsend and Robert Kennedy Jr., children of the late Sen. Robert F. Kennedy.
Robert Kennedy Jr. told ABC News, "Teddy and Caroline have made their judgment, and I absolutely respect those judgements I feel just as strongly about my reasons for supporting Hillary Clinton."
The Massachusetts senator made it clear he admires Hillary Clinton too. But his speech Monday afternoon was a blistering attack on the language and tactics of the Clinton campaign.
"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," Kennedy said Monday at the American University event.
Though the Kennedy endorsement carries obvious emotional significance for the Obama campaign and within the Democratic Party, its practical benefit is hard to gauge.
Potentially, it could weigh in Obama's favor come Feb. 5 in the Northeast and Southwest regions of the country, and with labor unions and Hispanics, the latter of whom come out in strong support of Kennedy's position on immigration reform.
But perhaps most significant at this stage in the delegate race for the Democratic nomination: a subtle reminder that a Democratic Party existed long before the Clinton era.
ABC News' David Wright contributed to this report.