"Sen. Barack Obama is a gifted and eloquent young man who I think 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."
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The convention hall resounded with boos when Lieberman said the Democratic presidential candidate voted to cut off funding "for our troops on the ground" in Iraq last year.
Playing his former party's spoiler, the Democrat-turned-Independent called McCain - not Democratic nominee Obama - the best choice to lead the country forward.
"What, after all, is a Democrat like me doing at a Republican convention like this?" Lieberman asked the cheering crowd. "Well, I'll tell you what, I'm here to support John McCain because country matters more than party.
"John McCain is the best choice to bring our country together and lead America forward," Lieberman added.
Appearing from the White House via satellite earlier in the night, President Bush hailed McCain as a man "ready to lead this nation," a courageous candidate who risked his White House ambitions to support an unpopular Iraq war.
Mr. Bush reprised the national security themes that propelled him to a second term as he spoke. "We need a president who understands the lessons of Sept. 11, 2001," he said. "That to protect America, we must stay on the offense, stop attacks before they happen and not wait to be hit again. The man we need is John McCain."
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"No matter what the issue, this man is honest and speaks straight from the heart," Mr. Bush said.
"The message of the day was designed to be about defining and re-introducing the candidate but the attention was focused on who was not in St. Paul - or at least not very visibly," said CBSNews.com senior political editor Vaughn Ververs. "Since being unveiled as perhaps the biggest political surprise in recent history last Friday, Republicans - especially the party activists gathered here at the Xcel Center - have been chattering aboutand little else. And the news media is not far behind in the curiosity category."
CBS News reports that Palin and former New York City mayor Rudy Giuliani will speak at the convention on Wednesday night during the 10-11 p.m. EDT hour.
Inside the convention hall, former Tennessee Sen. Fred Thompson delivered a strong defense of Palin.
Thompson said the Alaska governor, was "from a small town, with small town values, but apparently that's not good enough for some of the folks who are out there now attacking her and her family."
(Read of Thompson's speech and watch a
He said McCain's decision to place her on the ticket "has the other side and their friends in the media in a state of panic."
Thompson also took aim at the Democratic nominee Obama.
"Democrats present a history-making nominee for president. History making in that he is the most liberal, most inexperienced nominee ever to run for president," Thompson said as delegates roared their agreement.
Thompson jabbed at Obama on abortion, as well.
"We need a president who doesn't think that the protection of the unborn or a newly born baby is above his pay grade," he said in prepared remarks, referring to a recent episode in which McCain's White House rival said it was "above my pay grade" to decide the point at which an unborn child is entitled to rights.
Other Republicans - delegates and luminaries alike - defended Palin, who disclosed on Monday that her 17-year-old unmarried daughter is pregnant. In addition, a lawyer has been in an ethics-related controversy back home in Alaska.
"I haven't seen anything that comes out about her that in any way troubles me or shakes my confidence in her," said former Arkansas Gov. Mike Huckabee, who ran unsuccessfully for the party's presidential nomination this year.
"All it has done for me is say she is a human person with a real family," Huckabee said.
And Ron Nehring, chairman of the California state party, said video footage of Palin on a firing range was helping her cause.
"The reports I'm getting back is that every time they show that footage we get 1,000 precinct walkers from the NRA," he told members of his state's delegation, to laughter. "She cuts taxes and shoots moose. That's Gov. Palin," Nehring said.