Speaking Wednesday as the convention keynoter 12 years after he was keynote speaker for the Democrats in the same Madison Square Garden, Miller enthusiastically endorsed Republican President Bush.
"I have knocked on the door of this man's soul and found someone home, a God-fearing man with a good heart and a spine of tempered steel," Miller said. "The man I trust to protect my most precious possession: my family."
LIVE NOW: CBSNews.com webcast of the GOP convention.
Vice President Dick Cheney was to follow Miller at the podium on the third night of the GOP gathering. Cheney was also expected to launch a spirited attack on Kerry.
Cheney spokeswoman Anne Womack said the vice president will contrast Mr. Bush's "demonstrated leadership and decisiveness versus Senator Kerry's confusion of conviction – both in foreign and domestic policy – that he's demonstrated during his 20 years in the Senate,"
Cheney also planned to discuss the importance of public schools, a vibrant economy and improved health care system, Womack said, and will argue that these things are not possible unless the nation is safe and secure.
Miller was scathing in his criticism of his fellow Democratic senator, saying that Kerry had voted to cut weapon systems that the military needs to keep America safe. "This is a man who wants to be the commander in chief of our U.S. armed forces? U.S. forces armed with what? Spitballs?" Miller said.
Miller, a former Marine, also accused the Democrats of putting soldiers in danger just to get at the president.
"While young Americans are dying in the sands of Iraq and the mountains of Afghanistan, our nation is being torn apart and made weaker because of the Democrats' manic obsession to bring down our commander in chief," he said.
President Bush arrived in New York late Wednesday after attending a campaign rally in Ohio. He and first lady Laura Bush went to a community center for Italian-Americans in Queens, where Mr. Bush accepted the endorsement of the Uniformed Firefighters Association of Greater New York.
Kerry was in Nashville, Tenn., on Wednesday speaking to the same American Legion gathering that Mr. Bush addressed the day before. Noting that the president said earlier in the week that the terror war could not be won, Kerry said, "With the right policies, this is a war we can win."
In a television interview that aired Monday, Mr. Bush said the terror war couldn't be won. He amended the comment in his speech to the American Legion, saying it could be won.
Kerry's appearance broke a tradition in which presidential candidates have refrained from campaigning during the other party's convention. He used the occasion to again criticize Mr. Bush's Iraq policy.
"I would never have gone to war without a plan to win the peace," Kerry said.
As Wednesday night's program got underway, Republican delegates wrapped up the traditional roll call of the states, making President Bush their unanimous choice to lead their ticket in November.
The announcement brought a brief celebration on the convention floor – too brief for convention co-chair Michael Steele, Maryland's lieutenant governor.
He appealed to delegates, saying, "We can do better than that." He urged them to "bring it home for the president."
They responded with several minutes of cheering and the waving of signs reading, "Four more years" and "W, the president."
Delegates then made official Cheney's nomination for a second term as vice president.
Outside the convention hall, as thousands of people waving pink slips formed a line three miles long to protest the Bush administration's economic policies.
A small group of AIDS activists managed to penetrate Madison Square Garden itself, although the convention was not in session at the time. They blew whistles and chanted, "Bush kills," at a morning session of GOP youth before being hustled from the floor.
Police said convention-related arrests over the past week now totaled more than 1,700.
With two months remaining in a close election, and the pool of undecided voters a small one, Republicans relished the opportunity to place a Democrat out front at their convention. They had their man in Miller, a conservative ex-Marine who minces no words and delivered a keynote address a dozen years ago in the same hall in service of Democrat Bill Clinton.
"In this hour of danger our president has had the courage to stand up. And this Democrat is proud to stand up with him," the Georgia lawmaker said.
While Democrats are peeved at Miller for his speaking role, they plan to focus most of their criticism on Cheney.
"When Dick Cheney takes the stage at the convention, we will be reminding Americans about Dick Cheney's record, not only his abysmal failure as vice president but the sweetheart deals that his administration has cut with the company that he used to lead," said Matt Bennett, a spokesman for the Democrats. Halliburton, the company that was headed by Cheney before he became Mr. Bush's running mate, has a multibillion-dollar contract for work done in Iraq.
Cheney, 63, who served in President George H.W. Bush's administration as defense secretary, has seen his approval ratings plummet amid persistent questions about his role in promoting the Iraq war and in handling the Sept. 11 terror attacks.
But Mr. Bush has stuck fast by him, even as a new CNN/USA Today/Gallop poll conducted last week showed that 52 percent of those surveyed said they would vote for Democrat John Edwards for vice president over Cheney, if they could select the vice president separately.
Asked whether Cheney had given Mr. Bush good advice or bad advice over the past four years, 41 percent answered "good advice," 39 percent answered "bad advice" and 17 percent said they were unsure.