"That electrifying speech worried the Carter people, even though the votes were already counted [in Carter's favor]," Byrne says. "It showed how much passion Kennedy could arouse. That cemented his reputation in the party."
Along with his eloquence, Kennedy's commitment to a liberal legislative agenda -- pushing through bills on health care and labor laws -- made him a force to be reckoned with in the party, especially during his primary battle with Jimmy Carter in 1980 and jousting with Bill Clinton's centrist leanings in the 1990s.
To some liberal members of Congress, such as Rep. Barney Frank of Massachusetts, tonight's appearance will be a bittersweet moment.
"He's always inspirational, but it will remind people that he is ill," he says, describing the degree of sadness among liberals in reaction to his sickness.
"Teddy has been our conscience. And it's almost like a family where one sibling has been taking care of an aging parent and now that sibling will not be around. What do we do now? He's virtually impossible to replace -- who's there for the poor, the sick?"
Kennedy's words tonight expressed his commitment to public service, says Reed of Alabama.
"It is a message that will resonate," he says. "And one will expect that when a Kennedy speaks, the world is going to listen."
ABC News' Rick Klein, Jake Tapper and Nitya Venkataraman contributed to this report