"Sen. Barack Obama is a gifted and eloquent young man who can do great things for our country in the years ahead," Lieberman said. "But my friends, eloquence is no substitute for a record - not in these tough times for America."
The Connecticut lawmaker, who sought the Democratic presidential nomination in 2004, argued that Obama has not reached across party lines to achieve anything of significance and has been unwilling to take on Democratic interest groups.
In a clear appeal for backers of Obama's former rival Sen. Hillary Rodham Clinton, Lieberman said, "Let me contrast Barack Obama's record to that of the last Democratic president, Bill Clinton, who stood up to some of those same Democratic interest groups and worked with Republicans and got some important things done, like welfare reform, free trade agreements, and a balanced budget."
Lieberman said the comparison "may make history here at this Republican convention," but it drew only lukewarm response from the partisan crowd.
Lieberman addressed the second night of the Republican meeting just eight years after he stood before a cheering throng at the Democratic National Convention in Los Angeles and accepted the nomination as Al Gore's running mate.
He broke with his party over the war in Iraq and used his platform Tuesday to criticize Obama's national security record and hail McCain's. Lieberman, one of the strongest backers of the war, said that while Sen. Obama was voting to cut off funds for troops in Iraq, McCain took the unpopular position to support a surge in troops.
"Because of that, today, America's troops are coming home, thousands of them, and they're coming home in honor."
Seeking to reach beyond the GOP faithful gathered here, Lieberman said anyone who's ever contemplated voting Republican should do it this year.
"Tonight, I want to ask you, whether you are an independent, a Reagan Democrat, a Clinton Democrat, or just a plain old Democrat: This year, when you vote for president, vote for the person you believe is best for our country, not for the party you happen to belong to," Lieberman said.
Lieberman also sought the Democratic presidential nomination in 2004, but lost badly to John Kerry. Two years later, he lost a Democratic nomination for another term in Connecticut in 2006, then recovered quickly to win re-election as an independent.
His vote in the Senate gives the Democrats a narrow majority, but these days, he often calls himself an independent.
Former New Jersey Gov. Thomas Kean predicted Lieberman's speech would be the most interesting of the night, and shows tremendous courage.
"He's going to be punished by the Democratic Party and he knows it. But he wants to do it because he thinks he's the best candidate for president," Kean said.
Lieberman was also believed to be among those on McCain's vice presidential shortlist before the candidate selected Alaska Gov. Sarah Palin last week. Bedrock Republicans reacted strongly against the political moderate.
Lieberman's speech also echoed the McCain campaign story line about Palin: That she is a Washington outsider who courageously took on fellow Republicans in Alaska.
He painted McCain in a similar light, saying he's not "just another go-along partisan." Instead, he took on corrupt Republican lobbyists, big corporations and powerful colleagues in Congress.
"That's why I sincerely believe that the real ticket for change this year is the McCain-Palin ticket," he said to cheers.