"The same people who let this nation drift are trying to convince us that the last eight years have been squandered. Can you believe it?" Mr. Lieberman said at a cheering noontime rally.
"'They must go' is right," the Connecticut senator continued, answering chants in the crowd. "It's unbelievable."
Gov. George W. Bush, meanwhile, said Mr. Lieberman shared some of his positions and therefore put the Democrats in a tough spot.
Introducing Mr. Lieberman on a sweltering day on the plaza of Nashville's War Memorial, Mr. Gore addressed the history of the moment â€“ his selection of a Jewish running mate, the first ever on a major national ticket.
Forty years ago, the vice president said, Democrats tore down an "old wall of division" and nominated John F. Kennedy, who became the nation's first Catholic president.
"When we nominate Joe Lieberman for vice president, we will make history again," Mr. Gore said. "We will tear down an old wall of division once again."
He hailed the senator as a man of experience and integrity, one who he said would join him in fighting polluters and "big oil companies" and press for reforms in Medicare and for a "real Patients' Bill of Rights."
But he left the most stinging partisan rhetoric to Mr. Lieberman, who eagerly engaged the Republican ticket of Mr. Bush and Dick Cheney, the former Wyoming congressman and defense secretary.
Mr. Lieberman called last week's well-scripted Republican National Convention in Philadelphia mere "happy talk," asserting that Americans know very well that the Clinton-Gore administration has not squandered the last eight years, as the Republicans charged.
"Are we going to elect the old guard that created the problems, or a new guard that will continue to solve America's problems and make this a better, safer, more secure society?" Mr. Lieberman asked.
Borrowing the gist of a line that Mr. Bush's father used in his successful 1988 campaign for the White House, Mr. Lieberman asked, "If you have to change horses in midstream, doesn't it make sense to get on the one that's going in the right direction?"
And answering a new thunderous round of cheers and applause, he said, "Oh, I love that line. ... It's the truth."
Mr. Lieberman also dismissed assertions from the Bush campaign that "George Bush and I think alike."
"With all due respect," the senator said, drawing more applause from the clearly partisan crowd, "I think that's like saying that the veterinarian and the taxidermist are in the same business â€“ because, either way, you get your dog back."
It was a joke that Mr. Gore often tells on the campaign trail, and he laughed heartily at it. Both men, shedding their suit coats because of the heat, were obviously in high spirits, sometimes hugging each other and exchanging high-fives.
In Austin, Mr. Bush issued a statement praising Mr. Lieberman for "his convictions, his strong faith, and his record on Social Security, missile defense and reforming our public schools" â€“ all areas in which the Bush campaign says Mr. Lieberman is sometimes more in tune with Mr. Bush than Mr. Gore.
"This selection now presents the vice president with an interesting test of whether he will continue attacking positions his running mate shares, or whether he will lift up our nation by elevating the tone of his presidential campaign," Mr. Bush said.
During the rally, Mr. Gore, Mr. Lieberman and their wives spoke admiringly of their families and family values.
Tipper Gore, introducing Hadassah Lieberman, recalled how Mrs. Lieberman's parents had survived the Nazi concentration camps of World War II and immigrated to the United States.
"To me, she's not only a friend, but she represents what America is all about," Mrs. Gore said.
Responding in kind, Mrs. Lieberman said, "Whether you and your family immigrated from Europe, Africa, Mexico, Latin America, or Asia, I am standing here for you. This country is our country.
"This is your land, and anything is possible for us."
Her husband, again calling his selection a miracle, praised Mr. Gore for choosing an Orthodox Jew and breaking "this barrier in American history."
"It shows Al's faith in the tolerance of this diverse nation, in the basic fairness of the American people," Mr. Lieberman said. "I think we can say with certainty here in Nashville that the American dream is alive and well.
"You might almost say," he said, "that you could call this ticket the American Dream Team."
In the series of carefully choreographed speeches Tuesday, the Democratic Gore-Lieberman ticket sought to contrast itself starkly with the Republican Bush-Cheney team. And Mr. Lieberman, in particular, praised Mr. Gore as a man of character and faith.
"He has never, never wavered in his responsibilities as a father, as a husband and, yes, as a servant of God Almighty," Mr. Lieberman said.
The two-term senator, sometimes known as the conscience of the Senate, was one of the first prominent Democrats to rebuke President Clinton two years ago for his relationship with former White House intern Monica Lewinsky. And campaign aides are counting on some of Mr. Lieberman's moral authority to help separate Mr. Gore from Mr. Clinton as the campaign heads into the crucial fall finale.
So far, Mr. Gore's campaign chairman, William Daley, said reaction to Mr. Lieberman, from both Democrats and Republicans, has been much more positive than anyone in the Gore camp had anticipated.
Even his hard-to-impress brother, Chicago Mayor Richard M. Daley, couldn't "believe the reaction," the campaign chairman said.
"It is pretty amazing."